Becoming Elizabeth: Princess Mary will eventually become the queen known as Bloody Mary, thanks to her brutal executions of Protestants. However, when the series begins, she's the isolated Catholic in a mean-spirited Protestant court. She loves her siblings and her country, and she wants what's best for them. She tries to look out for Elizabeth, even though she has a Holier Than Thou streak that we know will turn very dark.
The show explores how Saul Goodman and Mike Ehrmantraut started their criminal careers years before running into Walter White. Its predecessor Breaking Bad is NOT an example of this, since Walter White was not introduced in a previous series and is instead a straightforward Protagonist Journey to Villain.
A flashback finally explained why Mike Ehrmantraut "broke bad". His son, also a police officer, was reluctant to take bribes and was executed by his crooked partners. Mike murdered them in revenge, then became a criminal to earn enough money to support his widowed daughter-in-law and granddaughter.
A flashback seems to show the moment that set Jimmy on his path to immorality, when he watches a conman bilk his father out of some cash. Determined to be a wolf rather than a sheep, the young Jimmy steals money from his father's till for the first time.
On Boardwalk Empire Nucky Thompson's is giving Gillian over to The Commodore; it's mentioned a few times but season 5 really drives this trope home by having a series of flashbacks showing Nucky's life building up to this point.
In season 5 of Buffy the Vampire Slayer, Spike gets a Start of Darkness episode called "Fool for Love".
Anya got one in season 7, although it's revealed her base personality hasn't changed as much as you'd think.
The books and comic books have reenacted Drusilla's Start Of Darkness at least twice. Since it involved Angelus murdering her family and torturing/raping her until she lost her mind, this is usually done to elaborate on Angelus' guilt.
Drusilla got a flashback in "Becoming Part 2" as well as the episode "Dear Boy" on Angel showing her as a human the time Angel started stalking her.
Played for Laughs with the Trio, who get a ten-second flashback origin story. "You wanna team up and take over Sunnydale?"
Cobra Kai, the Distant Sequel to the 1980s The Karate Kid films, gives one to John Kreese, the Evil Mentor of the first film'sThug Dojo. A series of flashbacks during the third season shows that once upon a time Kreese was just another kid struggling under difficult circumstances much like the various protagonists of the films, (his mother had suffered from mental illness and then killed herself when he was young, he had to work menial jobs to make ends meet and was bullied by and alienated from his peers), and when he met a nice girl he decided that going into the Army was the best way to make something of himself. Then Kreese made the mistake of volunteering for a special forces unit led by a sadistic and ruthless captain who taught him martial arts and a "No Mercy" philosophy, Kreese's fiancee died in an accident while he was at war, and finally he was captured and tortured by the North Vietnamese troops, which included being put into death matches against other captured soldiers. By the time he's rescued from captivity, the sympathetic teenager we first saw has turned into a brutal, abusive, amoral, and deluded man.
The Cold Case episode "The Woods" explores the background of a Serial Killer who made his debut earlier in the season.
The Criminal Minds episode "No Way Out Part II: The Evilution of Frank" has the team delve into the past of Frank, who had appeared in a previous episode and was described by Gideon as "the most prolific serial killer ever".
The Master gets a some of this in "The Sound of Drums". It's revealed he was driven insane during an Rite of Passage on Gallifrey, involving staring into the Untempered Schism, a gap in the fabric of reality which exposes the time vortex. This then expanded upon in "The End of Time", where it turns out Rassilon, Lord President of Gallifrey during the last days of the Time War, retroactively drove the Master insane on his own command to establish him as a link between Gallifrey inside the time-locked Time War and the tangible universe outside it.
The 2012 Christmas Episode"The Snowmen" unexpectedly turned out to be an origin story for 1960s villain the Great Intelligence.
Get Shorty: A Flash Back toward the end of Season 1 reveals that Amara was sold as a teen to a cartel boss for four goats to be his bride, but she killed him before he could exercise his Marital Rape License. She would go on to become a cartel boss in her own right.
Goodbye My Princess: Cheng Yin becomes increasingly vengeful and cruel after he learns who killed his mother and Cheng Ji.
Gotham is actually as much a Start of Darkness story for much of Batman's Rogues Gallery as it is, an expanded Origin Story for Batman. The Riddler, Mr. Freeze, Firefly, the Executioner, the Red Hood gang, and Scarecrow have all undergone seminal villain-making experiences as of Season 4's premiere, and future Gallery members like Selina Kyle, Ivy, Tommy Elliot and Harvey Dent have made major or minor appearances. Several who were already villains (Cobblepot, Butch, the Electrocutioner) develop their characters and/or technology further towards their eventual personas. The Joker's start of darkness was teased with several fakeouts before settling on Jerimiah Velaska getting sprayed with psycho serum by his very Joker-like twin brother and becoming a very cold, calculating and ingenious version of the villain.
The episode "Six Months Ago" shows how Sylar first killed a man and stole his power. Two seasons later, another flashback from the same moment in Villains expands the story and shows Elle and Noah Bennet are at least partially responsible for him becoming a serial killer. Especially since Elle stopped Sylar from committing suicide by hanging in the first place.
The Volume 2 story arcs in feudal Japan show the Start of Darkness for Adam Monroe, and some arcs in the axillary graphic novels show the background of characters like Thompson and Linderman.
In Volume 4, we also get the background for Angela Petrelli as well as the beginnings of The Company.
Holocaust has this for a major plot thread with Erik Dorff, initially a man of conscience who joins the SS at the urging of his opportunistic wife, is ordered to oversee a Nazi death camp and eventually slides into becoming a monster that only Heinrich Himmler could admire.
Barney in the first season episode "Game Night" of How I Met Your Mother. We learn how he evolved from a long-haired hippie guy into the Barnacle. Deconstructed, when we learn that a fair few of his issues stem from his crappy childhood, so this story, really, is about his tipping point.
Impulse: "The Moroi" is fully committed to exploring Nikolai's backstory. He was a poor Romanian teen who fled in the country's brief civil war sparked by communism falling, connecting later with his future adoptive family by befriending their son in Romania, committing his first murder while avenging his friend's murder before turning into the fearsome teleporter we know today, kidnapping others like him for cruel experimentation.
Dan Kuroto was a Child Prodigy game developer for GENM Corp., coming up with amazing ideas for brand new video games. Then one day, he got a letter from a little boy named Hojo Emu, who was so inspired by Kuroto's games that he came up with new ideas of games that he hoped Kuroto could develop. The idea that a child could even think of making games as good as him left Kuroto incensed, so much so that he decided to send a beta version of one game that he had been working on, albeit one infected with Bugster viruses, thus making Emu Patient Zero of the game disease epidemic.
The events of Zero Day were this to Taiga Hanaya/Kamen Rider Snipe. He was the first rider to fight the Bugster virus, but this was hopeles effort from the start as Kuroto had stacked the odds against him as much as possible, ultimately causing him to fail to save a patient and have all the blame pinned on him. Past the Despair Event Horizon he is a selfish jerk who doesn't care about anyone or anything past his goals. Kamen Rider Snipe: Episode ZERO shows the process in agonizing detail.
Liar (2017): Andrew's is revealed to have been when he'd discovered another serial rapist. Instead of turning him in, he blackmailed the man for lessons about it, as he's fascinated by the idea of control over other people. He then becomes a serial rapist himself under the first man's tutelage.
The Man in the High Castle: John Smith is established as a high-ranking Nazi officer in his very first scene, but Season 4 includes a significant flashback subplot depicting the moment that Smith decided to become a Nazi. Smith was a Major in the U.S. Army when the Germans nuked Washington D.C. and wiped out most of the political leadership. As Reich forces quickly took over the American seats of power, Smith sold his soul for a favorable position in the new order, even allowing his best friend to be shipped off to his death because he was a Jew.
My World… and Welcome to It: When John forgets to take Lydia to a picnic as he promised in "Child's Play," he imagines a multi-part Fantasy Sequence in which she has spiraled absurdly out of control later in life because of his thoughtless act — first becoming grossly obese from overeating to compensate for hurt feelings, then turning into a jailed pot-headed hippie, and finally morphing into a prostitute.
In order to prevent his son to be drafted into the ogre war, Rumpelstilskin gained magic powers by killing the Dark One.
Subverted in the episode "Sympathy For De Ville". Cruella is revealed to have been locked in an attic for years, seemingly under the thumb of an abusive Black Widow for a mother. However The Reveal is that Cruella was the murderess who killed her mother's three husbands—and it's implied that she had been evil all along. However the episode is still a straight example for Isaac—as meeting Cruella inspired him to use his powers to become a Reality Warper.
Mother Gothel had a horrifying one as well—she's a Plant Person, who as a young woman was curious about the human world and thought she'd made friends with a human noblewoman, only to have the noblewoman both humiliate her and recruit the local Anti-Magical Faction to find and eliminate the nymphs, leaving Gothel as the Last of Her Kind with a grudge against humanity itself.
For Carl Elias, it was his Mafia boss father killing his mother, and then trying to kill Elias.
For Elias' right-hand man Scarface, it was his abusive father beating him and his mother; one day Scarface snapped and murdered him.
For Samantha Groves (super-hacker "Root"), it was when her childhood friend Hanna was murdered and an adult she respected covered up for the killer. One of her first known forays into hacking was done to visit Laser-Guided Karma on the killer. Notably, when Finch wonders what her start of darkness was, she chides him for his attempt at kitchen psychology and denies that anything happened to her to make her the way she is. Whether this means Hanna's death had no effect on her pre-existing tendencies or whether she denies the effects to establish a distance from a buried trauma or whether she just means that nothing happened to her is unknown.
For Peter Collier, it was having his brother detained without charges and Driven to Suicide because a man he knew from his Alcoholics Anonymous meetings resembled a terrorist recruiter, followed by the officials responsible never getting punished.
Greer was an up and coming MI6 agent until he learned that his immediate superior was a double agent who burned a close friend of his.
Dominic is unique among POI arc villains in that he doesn't have a Start of Darkness episode.
Odin Quincannon is a coldhearted, borderline sociopathic businessman who can do pretty much whatever he wants (up to and including murder) in Annville courtesy of being practically the only employer in town. The episode "El Valero" eventually reveals what drove him to this — after the tragic deaths of his entire family in a cable car accident, he had a Freak Out and dismembered their bodies, finding that there was no difference between them and one of his cows, judging them as "just meat", and that the soul is a lie. This drives him to nihilistic atheism and madness, as he's determined to force everyone else to accept his own views.
A series of flashbacks throughout the first season chronicle the origins of the cowboy who will become the Saint of Killers that he himself is forced to relive in Hell. It sticks pretty close to the comic version.
Season 1 of Carnivàle delves heavily into Brother Justin's Start Of Darkness, as he develops from a compassionate and deeply moral laypriest into (at the very least) an unrepentant demon. It doesn't help that this was his bloodline and his sister murdered a bunch of orphans to engineer a personal tragedy for him.
Timeless reveals right away in the pilot that the reason Garcia Flynn is trying to completely screw up history is something to do with Rittenhouse, but despite hints dropped here and there over the next several episodes, it's not until "The Watergate Tapes" that we find out exactly why. As a contractor for the NSA, Flynn noted several large money transfers between Rittenhouse and Mason Industries (to fund their Time Travel experiments). Four days after he reported this to his superiors, Flynn's wife and daughter were murdered by home invaders, while he barely escaped. And since he doesn't know who in Rittenhouse ordered the hit, he's just going to Ret-Gone them as a whole, even if it destroys the United States as we know it.
Each of the Big Bads on Warehouse 13 get an episode (or details spread out over several episodes) explaining how they became the villains we know them as:
MacPherson used the Phoenix artifact to save his lover; by "dying" temporarily, he saw the afterlife, which from his point of view was nothing but darkness. He assumed this meant that there was nothing after life, and that all that matters is the now.
H. G. Wells lost her daughter, and started seeing only the worst in people; when she was de-Bronzed in the present, she saw things had only gotten worse and was pushed straight into Omnicidal Maniac territory.
Walter Sykes was corrupted as a child by an artifact that let him walk; when it was confiscated by Warehouse agents (specifically, Pete's mom), he became obsessed with getting it back and getting revenge on the Warehouse for taking it away.
Season 4 turns out to be one long SOD for Artie as his Enemy Within (manifesting as a hallucination of Brother Adrian) slowly consumes him.
Wentworth: In a series that acts as a reimagining/prequel to a show about a ruthless women's prison and the top dog that rules it, this is almost inevitable, but in Season 2 Episode 1 we finally see it. Bea overcomes her drug dependency, but takes a final sedative so she can visit her murdered daughter, Debbie, one last time in a hallucination. She says her goodbyes and assures Debbie that she has a new purpose now. Then she stares down Debbie's murderer as the warm light of the idyllic scene is replaced with the cold green-gray of prison and says, "I'm gonna kill the fucker."
Several non-consecutive episodes of Xena: Warrior Princess dealt with how Xena became the bloodthirsty warlord that she is trying to atone for.
