Buffy the Vampire Slayer has Spike — so infamously that the trope itself was once named "Spikeification."
He began the series as a straight villain who was set up to be killed off. Prior to this, he did things like kill the Annointed One, lead three powerful demon bounty hunters, and (once it was decided not to kill him off) betray Angelus behind his back and help save the world for his own selfish benefit. As the character became popular and got strung along throughout the rest of the show's run, he gradually became more and more sympathetic and cuddly. The change was so infamous that this trope was once called Spikeification. Despite his decay, the character would occasionally receive a few awesome moments to keep him interesting, and he wound up becoming somewhat badass again on Angel. The decay probably began around the season three episode when he stumbles back to town a heartbroken drunk after Drusilla broke up with him for not being evil enough. Thankfully, by the end of the episode, he realizes that all he needs to do to win her back is go back to being the person he was, i.e. a complete badass with a healthy dose of sociopathy.
The decay hit the ground running in season 4 when he is captured by the Initiative (a government paramilitary group studying demons) and has a chip installed in his head that prevents him from hurting humans. With the ability to cause harm removed, he is forced to become Angel-like: drinking blood obtained from butchers or blood banks, helping the Scoobies because he is so raring for a fight that he chose to attack other demons, and generally becoming angsty about his inabilities. However, it is when he falls in love with Buffy that he dives face-first into the realm of decayed.
This decay is justified out-of-universe, because to keep Spike on the show past his time as a villain, he would have to be rendered a non-threat, so the audience wouldn't be wondering why Buffy didn't just stake him. Subsequent episodes where he proved that he was still a threat - either indirectly (e.g. setting her up to be killed by Drusilla in "Crush") or directly (e.g. that scene in "Seeing Red") - but still didn't get staked left the base permanently broken.
Also, the Turok-Han. When it was first introduced, the "ubervamp" was so powerful that it just couldn't be stopped by any member of the Scooby gang and it almost killed Buffy. Twice. On the third try, Buffy kills it while giving a speech to all the Potentials and the Scoobies and Andrew in a Moment of Awesome, but just barely. In "Chosen", after Willow turns all Potentials into Slayers, you can see a whole army of ubervamps go down like flies, as Buffy's army kill them as if they weren't stronger than regular vampires. And it's not only the power of a slayer; Robin and Giles and even Anya are seen killing a few of them!
Irina Derevko from Alias. Even Sark showed hints of humanity, but Derevko was given the Villain Ball in season 5. Sark at least stayed believably unredeemed.
Chuck: To an extent, Shaw. He started off as a super spy - smart, tough, and skilled. All those skills were enhanced even further when he got the Intersect. However, after he lost the Intersect, Chuck managed to hold his own in a fight against him for a while. This could be partially attributed to Chuck's increased skills, but Shaw should have been able to do better.
An In-Universe example from Brooklyn Nine-Nine. Hitchcock and Scully are a pair of lazy, incompetent slobs who are basically just riding it out until retirement. However the season six episode "Hitchcock and Scully" showed that, back in the eighties, they were known as "The Studs of the Nine-Nine," and were a pair of handsome, intelligent, athletic supercops. They were known as "Flat Top and the Freak" and were so well-respected that they'd walk into the precinct after a major bust, fire off a snappy one-liner, and get applause from the other officers and detectives.
Young Scully: Hey! Anybody have a trash can? Because Flat Top and the Freak are bringing in some garbage! Nine Nine officers and detectives:[applause] Jake: You guys had badass entrance lines and people cheered?! What happened?! Where'd it all go wrong?
Invoked about the Night's Watch, which has fallen on hard times. It was once a highly-regarded order on par with the Order of Maesters, attracting the best and most elite of Westeros. However, in later centuries, of the veritable army that once existed to safeguard the realm, only a token militia of less than a thousand men remains, who are mostly the bastard or unfit sons of Lords, along with various criminals, rapists, or those who fought on the wrong side in a war, who join in order to receive a full pardon. They are also severely under-equipped and can only afford to keep three of their nineteen castles along the Wall manned and maintained.
Evoked and discussed in-universe with Robert Baratheon, who was a mighty warrior. When the series begins he's living on past glories, is too fat for his armor, and spends his days partying and trying not to piss himself.
The Kingsguard was once admired as the finest examples of chivalry and combat skill in the realm. Now they are mostly comprised of mediocre fighters and unchivalrous brutes.
Jaime goes through this In-Universe as from the beginning of the series onwards he is captured, out-fought by a woman, and finally crippled, losing all of his prodigious skill as a swordsman. By Season 4, even his own father and Cersei regard his career as a Knight as over and Joffrey essentially calls him a has-been.
Invoked by Littlefinger regarding the Lannisters; with Tywin and the sheer power of his will gone, all that remains is one-handed Jaime, weak-willed Tommen, and dowager queen Cersei.
In-universe with the people from the Iron Islands. Once, they had been the most feared people in Westeros. They had basically conquered the entire Riverlands, which involved beating the shit out of the Stormlords. Then they successfully defeated and drove back the Andals, eventually subsuming them into their society and not the other way around as in the rest of Westeros (even the North's been affected to a certain extent by Andal culture in a way the Iron Islands never have), and built the single strongest fortress in the continent that was only brought low because of fucking dragons! And according to legend one of their ancient kings slew a sea dragon. Things went downhill after Aegon the Conqueror burned Harrenhal and helped the river lords drive their ironborn overlords back into the sea. Balon Greyjoy tried to reverse this by seceding from the Seven Kingdoms, but this backfired spectacularly when the Baratheons and their allies beat them into submission.
Varys. At the beginning of the show, he was a chessmaster and spymasterpar excellence. By Season 7, however, he's just...there. His intelligence network must either be composed of amateurs or just nonexistent, because it failed to notice tiny little details like Euron Greyjoy's fleet attacking Yara and Theon's ships, and then also slipping up on the fact that House Lannister had abandoned Casterly Rock and was marching on Highgarden, the seat of Targaryen ally House Tyrell, and also the aforemntioned Euron sailing to Casterly Rock to cut off Dany's forces there. Possibly justified, given that Qyburn might have taken over Varys' spy network. On the other hand, that shows a very dangerous lack of foresight on Varys' part. Also a case of Adaptational Wimp, given that, in the books, Varys still has his "little birds", and uses them to kill Kevan Lannister.
Fellow genius and master of intrigue Tyrion has much the same problem. In the first few seasons, he adroitly handled himself in a Decadent Court despite being viewed as an invalid and displayed excellent prowess on the battlefield, as well as a cynical awareness of how cruel the world could be. Starting around the seventh season, it seems like the easiest way to determine whether a plan is going to fail is to check and see if Tyrion was involved in its conception. Nearly every battle he's put in charge of is a Pyrrhic Victory at best, despite the strength of his army, and his attempts to guess Cersei's moves are repeatedly, ridiculously off.
Arthur Fonzarelli, "The Fonzie" from Happy Days, is first flanderized and then decayed through the course of the series, but especially after it jumps the shark. He ends up being more like a Boys' Club leader than the aloof, antisocial cool guy he started the show as.
Cylons in general have avoided badass decay, but Caprica-Six seems to have suffered rather badly. She went from baby mercy-killing in the miniseries to pining for Baltar and desiring co-existence with humans in Downloaded, though it was clear she cared about Baltar in the miniseries and she wasn't seen again until Downloaded anyway, so its not as if we had much evidence of badassery on her part anyway. That at least led to the scary occupation of New Caprica. After that, unfortunately, she was eventually reduced to surrendering along with Baltar, and sitting in Galactica's brig getting hardly any screen time. It's well-written legitimate character development, up until late season 3 where the writers almost forgot she existed for a time.
Starbuck goes through is a little bit in the later seasons, especially when she dies and comes back. She goes from being the badass Viper pilot to captain of a reconnaissance mission. The biggest sign of this is when she slaps Baltar for revealing a secret about her. In previous seasons she sucker-punched both Tigh and Apollo when provoked, but here she fights like The Load.
Arguably done in The Plan, so as to humanize or explain the motives behind the Cylons' titular plot to destroy Galactica. Instead of resorting to outright warfare, the few human Cylons left in the fleet have to resort to cloak-and-dagger tactics while re-evaluating their role as villains. Simon goes from the scheming, mastermind doctor in "The Farm" to a loving family man who would rather kill himself than blow up a ship with his human family on it. Cavil becomes an Iron Woobie who laments the continuous failure of his plans, while the Caprican variant of himself eventually becomes disenchanted with their actions, preaches harmony and unity and gets boxed by his fellow model.
Although never exactly a badass, Norman Clegg from Last of the Summer Wine began the long-running series as an acerbic philosopher with a dry and pointed sense of humour (as well as functioning as the Ego of the series' Power Trio). As time has gone on, however, he has become a total wimp: scared stiff of driving cars, terrified of the various female characters (especially Auntie Wainright) and increasingly resigned to whatever madcap scheme his current "leader" has in mind.
Lord Zedd in Mighty Morphin' Power Rangers began life as the "Emperor Of Evil": a genuinely terrifying villain who quickly banished the comical Rita Repulsa and proved his magnificence by almost destroying the Rangers' zords, finally stripping Tommy of his Green Ranger powers, and creating his own highly kick-ass zord Serpentera, which towered above the Rangers' own Megazord. Then near the end of the 2nd season he got married to Rita, becoming a more comical villain as the show went on and by the third season the transformation had become complete and he was a Jaded Washout type character. By next season in Zeo he along with the rest of the Morphin baddies are run off by the Machine Empire (His voice actor confirms "Zedd scared small children, so they invoked this trope".) Ironically, he seems to do better in season three, as he pulls off more evil schemes which may convince the audience that he might even win. How? One of his plans climaxed in entering the Command Center, throne and all.
In the first season, Goldar was The Dragon and more than a match for the entire team of 5. Jason was capable of trading blows with Goldar, but that was it. But with season two more about Tommy than any other cast member, Goldar's character suffered dreadfully. Tommy began to defeat Goldar singlehandedly, but it wasn't just that Tommy's skills were growing. In one episode, Billy was able to kick Goldar around, unmorphed, and that's when it got depressing. One might expect he'd be furious and try to regain his honor, but instead he just turned into a bungling nincompoop.
Toyed with and retconned several times in Heroes with Sylar. Several storylines make it look like he's becoming a more sympathetic character until the arc is aborted and he snaps back to being a psychopath.
When the Time Lords first show up at the end of the Second Doctor's run, they were mysterious, powerful, and threatening to the Doctor. They forced him to regenerate and banished his regenerated self to Earth. Over time, the portrayal of Time Lords changed into that of a stagnant society in decline, who had largely forgotten much of their former power and morality. The downgrading of the Time Lords happened first accidentally and then deliberately. In "The Three Doctors" they appeared as more Human Alien than god-like. Then when they re-appeared in "The Deadly Assassin" they fell prey to deliberate Take That!Retcon by writer/script editor Robert Holmes, Armed with Canon. Fandom at the time complained about Holmes' story, but it established the trend which later writers took and ran with.
The Brigadier, when he first appeared in the late 1960s (real time) took no guff from anyone and the stories portrayed UNIT, the force he led, as an elite team of defenders against Alien Invasion. He got gradually more comedic and less impressive, though he would regain his reputation later. As did UNIT itself.
The Cybermen in the classic era were infamously subject to this. By the time 80's Who rolled around, the Cybermen were routinely being killed with solvents, concentrated fire from Human weapons and most infamously, anything made of gold, from gold dust to gold coins. This appeared to have been rectified in the New Who era. However, later on, they are defeated by a golden ticket and a father's love for his son.
The Sontarans suffered a minor version of this-they were almost unarguably more dangerous in the 1970s and 1980s, at one point invading Gallifrey itself, but become a little less badass and rather more comic in stories such as "The Sontaran Stratagem" and "Time of the Doctor." However, they're still quite dangerous, so it's more of a downplayed trope.
In the fourth season of 24 Curtis Manning was a pure badass, so much that he was called Black Bauer. In the fifth season he was mostly a doormat compared to Bauer, in the sixth season he was ineffectual until he got killed by Bauer.
Tyr Anasazi of Andromeda went from being one of the show's best Magnificent Bastards and the only mortal being in the universe that Dylan Hunt couldn't take in a fight to a driveling short-sighted idiot that ended up losing fights to all and sundry, and was ignominiously shot in the back and dropped off a cliff. Some viewers believe that Kevin Sorbo (who played Dylan Hunt) becoming executive producer might have had something to do with this.
The main character almost succumbed to this in season two, even going so far as planning to turn himself in as the Bay Harbor Butcher, but thankfully changed his mind. As the series went on, Dexter has suffered from Badass Decay anyway. After all, each season charts a new step of emotional development for Dexter, as he discovers that he's not quite as inhuman as his adoptive father led him to believe. As a result, the Dexter at the end of Season 5 is not nearly as dark as his Season 1 counterpart. The series veered in a new direction in Season 7, with Dexter willfully rejecting the Code of Harry and becoming more of a Badass.
Hannah suffered from this between season 7 and 8. Initially, she was a Dark Action Girl whose MO was to seduce victims into lowering their guard before poisoning them with rare poisonous plants she breeds in her garden. One of season 7's best moments has to be her murder of Sal Price in this fashion. She then gets Put on a Bus until mid-season 8; upon her return, the audience was disappointed as she was reduced to a weak woman who apparently couldn't take care of herself anymore (which she had been doing since she was 14) and needed Dexter for everything, resulting in her being too dependent of him and eventually leading to a Romantic Plot Tumor. And the showrunners just couldn't get enough of her.
The Borg from Star Trek. In their first appearance they started carving the Enterprise like a turkey, Borg drones had a personal energy shield that would adapt to enemy weapons fire after other drones would fall, the Picard needed to beg Q for help just to let the ship escape. A single Borg ship (with Picard assimilated) was powerful enough to destroy 39 Federation spaceships in the battle at Wolf 359, break through the Solar System's defense grid and reach Earth orbit. Early on, writers realized that because the original Borg concept was so single-minded they needed to modify some concepts to make for more story potential. The Borg turned to assimilating both people and technology, instead of being their own unique race. Star Trek: First Contact introduced the concept of Borg "queens," which while effective for that movie, the Queen inherently humanized them, making simple deception a viable option to deal with them. By the end of Star Trek: Voyager, the Borg's bad-assness had decayed so badly that Janeway routinely blew up whole Borg cubes with just a mean stare. However, the Borg did roar back to badass level in the post-Nemesis novel continuity and Star Trek Online. How badass? Well, let's just say eating fucking Pluto was just the beginning.
In the '60s spy series The Man from U.N.C.L.E., heroes Napoleon Solo and Illya Kuryakin are sometimes subject to plot dependent Badass Decay. E.g., in the third act of the third season episode, "The Five Daughters Affair, Part II", Solo and Kuryakin fight THRUSH's "karate killers" (who despite that name (as given in the credits) do very little actual killing in the episode) for about the sixth time in the two-part adventure. Despite holding their own in several earlier fights with the karate killers, Solo and Kuryakin lose whatever fighting skills they've demonstrated, and are straightway handed their asses by the THRUSH "killers". This is necessary to set up the fourth act's climax and resolution (therefore "plot dependent").
Michael Caffee of Brotherhood goes from Badass Punisher-like Ear-cutting Irish Mob vigilante to brain damaged bagman then paranoid drugged mob-boss.
The titular House suffered from this in Season 6. Where once there was a badass, sarcastic, biting floating brain, there now stands a love-sick puppy that spouts "emotionally healthy" psychobabble. Most notable in his relationship with Cuddy, which went from mutual messing to confessions of love. Confessions. Of love. From House.
Lost had some serious decay with the Others. What started as a mysterious group of rogue jungle ninjas was soon revealed to be little more than a bunch of commune dwelling nobodies that played football and had a flare for the dramatic. Although shining the spotlight on anything scary will quickly reveal that it's just a branch scratching against a window. See Nothing Is Scarier.
Morgause from Merlin was simply too intelligent for her own good. In her first appearance she storms into Camelot, takes down several guards, challenges Prince Arthur to a duel, beats him, drops a bombshell about his mother, makes him chase her across the countryside for answers, shows him what may or may not be a real apparition of his mother who tells him that his father was responsible for her death, and then watches from a crystal as he goes storming back to Camelot to kill King Uther in a fit of rage. However, in a show that seems almost pathologically dependent on Status Quo Is God and pressing the Reset Button, Morgause is reduced in season three to a completely ineffectual and one-dimensional villain who plots to overthrow the kingdom with a range of increasingly convoluted plans. If she had been allowed to retain the intelligence and subtlety she had displayed in the second season, she would have been running the place in two seconds flat.
Barnabas Collins of the original Dark Shadows was intended to be the latest villain when he first appeared. His first victim was Willie Loomis (who was looking for jewels supposedly buried in the family crypt), turned into his slave. Jason McGuire, who had come to Collinwood to blackmail Elizabeth Stoddard, was one of his first on-screen kills (Jason had brought Loomis with him, and become interested in the jewels Willie sold for Barnabas). He kidnapped and tormented Maggie Evans in an effort to make her into a version of his first love, Josette; killed anyone who got in his way; manipulated Dr. Hoffmann's affection for him, and generally caused mayhem. Then audiences fell in love with him, leading to his transformation into a heroic character.
Eric of True Blood started as a fetishy ruthless powerful vampire with a certain human streak. Badass Decay was predetermined. He stayed Badass three seasons. At season 4 he's been cursed by a group of hobby esoterics, lost his memory along with his personality and has taken to making angsty confessions to Sookie. He seems to have gotten back his badassery by the end of the season, when he decapitates three heavily-armed guards in the space of a second, while Bill stakes the vamp who put him in power.
Young Blades: Pointed out in-universe when D'Artagnan discovers that his famous Musketeer father has been reduced to performing for money and selling action figures of himself.
Entourage had Ari Gold go from a hyper-ambitious, foul-mouthed, ruthless agent for Hollywood's A-list to a guy whose main accomplishment was securing a Hallmark movie-of-the-week role for a washed up TV actor.
Supernatural: Castiel made his debut as a badass Angel of the Lord who dragged Dean out of Hell, slaughtered an entire group of demons single-handedly, burned out Pamela's eyes when she saw his true form, caused a lightning storm just by appearing, and No Sell'dRuby's Knife. Fast forward a couple seasons, and he's been flanderized into a socially awkward, klutzy virgin. Justified in-universe for various reasons; Season 5 saw him lose a significant portion of his power due to his rebellion against Heaven, Season 6 saw him distracted by a civil war with the archangel Raphael, Season 7 rendered him briefly amnesic and subsequently insane, Season 8 had him torn between loyalties as his mind was manipulated while he recovered from being trapped in Purgatory, Season 9 and 10 saw him stripped of his powers and reduced to repowering himself by taking the power from other angels until his own was restored, and subsequent seasons pitted him against a range of powerful threats that limited what he could do against them.
Dr. Zachary Smith from Lost in Space, started as a cold, cruel, traitorous villain, to became the main comic relief as a cowardly, incompetent even effeminate burden.
After reaching three consecutive grand finals (being the only robot ever to do so), Hypnodisc fared dismally, knocked out in a Series 6 semi-final and defeating only fragile opponents in Extreme II.
Chaos 2 spent Series 3 and 4 Curbstomping all its opponents. Never upgraded or re-designed by engineer George Francis despite new weight allowances for Series 5, Chaos 2 was subsequently defeated by a newcomer in a Series 6 heat final and was thrashed in Extreme II.
Pussycat, possessing a powerful spinning blade and guided by David Gribble, considered by Julia Reed to be the most talented driver to have ever graced the show, was formidable enough to come second in Series 4 and even defeatedRazertwice. Tragically however David Gribble was killed in a motorcycle accident between Series 4 and Series 5 and Pussycat just couldn't quite deliver a repeat performance. Pussycat at least went out on a high note, beating three American robots to win in the War of Independence special.
Stargate SG-1 gives us the Harsesis — a child born with all of the knowledge of the Goa'uld's advanced technology. So much of a potential asset/threat was he, that an ascended being with powers approaching that of a god eventually had to take him under her wing for everyone's protection. The thing to note about Goa'uld tech though is that whilst it is hundreds, perhaps even thousands of years ahead of our own, it all amounts to the equivalent of stone knives and bearskins when put up against the technology of the Ancients and the Asgard. Thus when the Earth eventually gained a fleet of space ships powered by Asgard tech, and a flying Ancient city, and an Ancient ship that can jump whole galaxies in weeks, and complete databases of all of said races latest technology... lets just say that the Harsesis child is actually a bit underwhelming now.
Both Darlene and Dom suffer from this in Mr. Robot. Darlene starts off as a no nonsense and hardcore hacker who is Elliot's sister and seems to be brave at dealing with everything and Dom is a determined and confident FBI agent who despite her social anxiety manages to use her talents and intelligence as an FBI agent to take down criminal. The Dark Army machinations against them ends up turning both the women more broken and weak in Season 4 and leaving them unable to defend themselves against them. Though Dom manages to take down her psychotic handler and Darlene manages to take down the Deus Group who handled them.
