- Batman:
- This has been happening to Batman himself for a long time throughout his publication history and is arguably still happening now. Ever since the dark age of comic books began, writers have been increasingly portraying him as being more and more mentally unstable. Notable examples include him being ludicrously paranoid and spying on his friends with a giant spy satellite in space, or him literally shattering his own mind to create a split personality as a backup contingency plan in case he went insane someday, to outright deceiving and manipulating his friends and loved ones in a questionable manner and acting like a massive Jerkass Control Freak. Some stories even imply that he may be just as crazy as The Joker and that he belongs in Arkham Asylum as much as his nemesis. It’s especially jarring when you compare him to his early appearances in the golden age and silver age to his present self (granted, he was violent and wasn’t afraid to kill back in the golden age, but he was still more rational by comparison). Some stories even play it as a plot point that his endless war on crime has been taking a heavy toll on his sanity and that the longer it goes on, the more it destroys him mentally. Basically, the further his story progresses, the further his mind slips into madness.
- Azrael: Jean-Paul Valley's stint as Batman during the "Knightquest" portion of the Knightfall storyline went like this: After being doused with the Scarecrow's fear gas, the hypnotic trance known as the System kicked in, haunting Jean-Paul with images of his father and of Saint Dumas, the "creator" of the order he once was part of. As he's haunted by these apparitions, The System drives him to be more brutal and continuously modify the Bat-Costume, the pinnacle being when he allowed Abattoir to die, condemning his captive to death, modifying the Bat-Costume to the point where it was less Batman and more Azrael and outright kicking Robin out of the Batcave and working against him, alienating Commissioner Gordon, and Bruce Wayne and deciding that all criminals need to be put out of their misery. It's at that point that Bruce decides enough is enough and sets out to take back the Mantle of the Bat.
- In the Batman Vampire trilogy, although Batman is a straightforward hero in the first book, by the second book in the series he begins to lose his mind as he becomes increasingly tempted by his new need for human blood after becoming a vampire to properly fight Dracula, until he drains the Joker and has Alfred and Gordon stake him before he can hurt anyone else. Unfortunately, staking on its own just trapped Batman in a death-like state where he was aware of his decaying body but unable to move, leaving him even more deranged when Alfred removed the stake to restore Gotham's hero in the midst of a terrible crime wave, in a desperate hope that his master could still be a good man. Consumed by his thirst, Batman literally tears through his rogue's gallery to feed, each death affirming that the man he was is basically gone, culminating in the former hero ranting about how he will drain the blood of his old friend James Gordon if Gordon won't kill him first.
- The Killing Joke is one take on the Sanity Slippage that eventually led to the origin of the Joker. But the Joker admits he's an Unreliable Narrator and could be remembering wrong or making the whole thing up. He attempts something similar to crack Commissioner Gordon. He doesn't succeed.
- The Batman Adventures: This is what happens to Hugo Strange. By the end of Vol. 1's final arc, he's literally seeing his son's face in everyone he looks at, as a result of tampering with his own memories. He tried to remove the one of his son's death but wound up also removing the one of the memory-removal itself, thus he started obsessively removing every single memory he had. And it still didn't help one bit.
- In The Untold Legend of the Batman, a mysterious foe is attacking Batman, destroying his father's "Bat-Man" costume (in this point of Pre-Crisis continuity, it was one of the inspirations for Bruce) and destroying the Batmobile. Batman later heads to the abandoned Wayne Manor and is caught in a Death Trap where Bats realizes that he has been the one doing this, thinking he finally lost his mind. Robin saves Batman from the trap wearing a copy of the "Bat-Man" costume, getting him to understand he's only suffering lingering effects from getting caught in an explosion. The shock is enough to rattle Batman back to sanity.
- In The Attack of the Annihilator, Kenneth Anderson was merely an embittered jerkass of a scientist before being exposed to unknown alien energies. Whereupon being transformed into some kind of psychic mutant, he declares he has become a god, and starts blasting holes into buildings in revenge for being supposedly belittled by his peers. As his body continutes to mutate, Anderson -now calling himself "the Annihilator"- decides Gotham not only must be destroyed but also rebuilt and repopulated with his superior mutant offspring.
- In Convergence: The Atom #1, Pre-Flashpoint Ray Palmer has been hearing a voice in his head that may or may not be Ryan Choi's and has been acting considerably loopy ever since he was trapped in Gotham.
- Superman:
- In Supergirl (2011), Krypton's destruction and his inability to save Argo City and his own family have deteriorated Zor-El's sanity to the point he strikes a bargain with Brainiac of all people and becomes Cyborg Superman. Supergirl wonders what happened to his father after hearing about some of his actions in Supergirl (Rebirth).
- In Krypton No More, super-villain Radion got radioactive powers because of an accident. His sanity has been deteriorating since, and he has become a raving, murderous, megalomaniac lunatic.
- Superman vs. the Amazing Spider-Man During the climax, Superman thinks his ex-friend Lex Luthor was always about to lose it... and he's finally lost it.Superman: Luthor's slipping — the pressure is beginning to get to him! He's always walked a fine line between genius and insanity — and this time, I think he's taken the fatal fall!
- In A Mind-Switch in Time, Euphor feeds on massive quantities of negative emotions until they take a toll on his sanity, causing him to go from polite man whose desire to help people appears selfless and genuine to megalomaniac creep who brainwashes Metropolis into serving him.
- Strangers at the Heart's Core: Spending more than one decade as a disembodied, wandering ghost after getting murdered slowly and gradually wore Lesla-Lar's sanity down until she deluded herself into believing she was Supergirl's long-lost sister and became obsessed with destroying her.
- The Death of Superman revealed that this happened to the Cyborg Superman. The stress of being a disembodied energy being controlling machinery, his wife committing suicide upon seeing his robotic body and being alone in space caused Hank Henshaw to lose his mind and believe Superman drove him from Earth in jealousy.
- Superman vs. Shazam!: Apparently, Karmang was a good person before becoming immortal, but spending one million of years tortured by legions of ghosts has eroded both his goodness and his sanity and nowadays he is willing — and eager — to blow planets up in order to get whatever he wants.
- Wonder Woman:
- Wonder Woman (1987): After Circe recovers her memories following a Memory Gambit that left her with memories of liking humanity and befriending Diana she tries to rebel against the new things she'd acclimated to by allying herself with a new villain in what was presented as, to her, a little nihilistic throwback to old times, the next time she shows up with her own plan it's incredibly flawed and she's hardly paying attention to what she's doing. By the end of her confrontation with Wondy she is outright goading Diana to beat her to death and laughs without defending herself or trying to escape when called out on it. The implication is that she both hates and likes humans and can't reconcile her new feelings with her past (and present) actions.
- Wonder Woman (Rebirth): Diana herself starts losing it with the revelation that many of her memories are false constructs in #13 as her mind tries to rebuild itself while she's also dealing with a magically potent venom. She suffers from severe disassociation and hallucinations.
Vertigo Comics, WildStorm, Other Imprints
- Jackson Georges from Ex Machina, Mayor Hundred's former NSA handler fits this trope. In flashbacks the reader is shown how the cryptic shard that Mitchell found when he got his powers slowly drives Jackson to levels of extreme paranoia. He develops an unfounded contempt for Mitchell, claiming that without Mitchell's appearance and the mystery of the shard, he could have foreseen and prevented 9/11. He becomes obsessed with protecting his family from dirty bombs and the like, buying a Hazmat Suit and making tally marks on the wall to represent god knows what. It's clear he's a step away from the deep end. Darkly subverted when in desperation to save their failing marriage, his wife breaks into his work shed and steals the shard. A few seconds near a TV are enough to drive her completely and utterly insane, leading her to kill her daughter, husband, and dog. She even chops off her own arm with the shard itself. It was an incredibly jarring and brutal twist on the slow burn of paranoia the reader had been witnessing for months.
- The time between when he last fought the Justice League of America and his appearance in the opening arc of The Sandman (1989) wasn't kind to Dr. Destiny — while he wasn't a saint before, he's now a Psychopathic Manchild who got his jollies torturing a diner full of people.
- Rorschach from Watchmen is clearly mentally unstable even before the event that causes him to slide off of the slippery slope.
- The Avengers: Hank Pym has a bad dissociative episode during the Roy Thomas run, thanks to inhaling some mysterious gases in a lab accident. He becomes a new, antiheroic person called "Yellowjacket" and insists he killed Hank Pym. The Wasp sees no problem with this and even decides to marry Yellowjacket once she figures out he's really Hank, since she figures it's the only way Hank would marry her. The wedding is attacked by the Circus of Crime, and Yellowjacket reverts to Hank, but nobody figures it might be an idea to get Hank off to a shrink stat. Several years later, during Jim Shooter's run, Hank's mental state gets worse again for a variety of reasons. Much later than that, Hank would be officially diagnosed as bipolar.
- Captain America: John Walker, during his time as Cap. Initially starting off as a flawed replacement for Cap who, despite his disdain for Steve Rogers still respects the title, he accidentally beats a supervillain to death. Then his former sidekicks expose his identity to the world, leading a hate-group to abduct and kill his parents, at which point John starts to take a nose-dive. The Red Skull eggs this on, as part of his own plot to ruin Captain America's image forever. By the time Steve returns to take the title back, John has gone completely berserk in his grief, attacking Steve because he thinks he's an imposter, all with a demented smile on his face.
- Iron Man 2020 (Event) and its lead up are really all about Arno Stark's descent into this. His Jumping Off the Slippery Slope by having Tony declared illegitmate due to the nature of his resurrection, stealing Tony's business and identity as Iron Man, replacing Sunset Bain with a robotic duplicate — and all for a delusion due to his disease coming back and killing him.
- Spider-Man:
- In The Amazing Spider-Man (1963), J. Jonah Jameson suffered a nervous breakdown late in Marv Wolfman's run, to the point where he believed Spider-Man was stalking him. However, Wolfman left the book before long, and Roger Stern retconned the whole thing away as mind-control.
- In Peter Parker, The Spectacular Spider-Man #21, it's revealed that Mac Gargan's mental stability has hit rock-bottom and that he believes he's mutated into a scorpion monster. When he goes after J. Jonah Jameson hellbent on revenge, Spider-Man intervenes and defuses the situation by unmasking Gargan to show him that he's still human.
- Thunderbolts:
- Moonstone starts cracking more and more through Niceiza's run, until in Avengers / Thunderbolts she finally snaps and goes on a psychotic rampage, ending with her going into a coma that lasts until the aftermath of Civil War, when her moonstone is given back to her.
- After Civil War, Norman Osborn is put in charge of the Thunderbolts, who now work for the government and capture unregistered superheroes, his Ax-Crazy nature kept in control by medication. Every once in a while he is asked about his previous alter ego the Green Goblin. Every time that name comes up, Osborn begins to crack a little. Every time he hears the name Spider-Man he cracks even more. Also it doesn't help that his second in command Moonstone is gaslighting him (leaves his Green Goblin mask in his desk) and switching out his medication, just to drive him over the slope so she can get his job. It works. Perhaps a bit too well.
- Penance spends much of Warren Ellis's run getting steadily worse. A brief visit from Doc Samson looks like it was fixing this, but then other things happen. By his reappearance in Avengers: The Initative, Robbie's gotten far worse... and Norman Osborn explicitly orders his therapist not to help him in any way.
- The "Caged Angels" arc has this happen to pretty much everyone on the team, thanks to a group of captured psychics messing with their heads. Venom decides to eat everyone, Swordsman starts trying to act like his dad, Norman resumes being the Green Goblin and decides to kill everyone for fun, Moonstone goes to attack Doc Samson, and Radioactive Man decides he'll use his powers to give everyone cancer.
- The Transformers (Marvel):
- After Optimus Prime dies (the first time, that is), Megatron takes it poorly, and becomes convinced Optimus is still alive and planning something. His madness eventually winds up causing him to shoot at the Space Bridge, which promptly explodes and apparently kills him. Or not, as it turns out Megatron was just Obfuscating Insanity, and using his 'death' as a cover to return to Cybertron. He just miscalculated when shooting the Space Bridge, resulting in a spot of Laser-Guided Amnesia.
- Happens to Shockwave in the UK comics, when he learns he dies in the future. Determined to prevent this, he finds Megatron and sets him on Galvatron (well... it's complicated). Then the two start working together. Shockwave, who didn't expect this, just snaps, and kills anyone who approaches him.
- Ultimate X Men: Mutants have a big problem being accepted by regular humans as is, but Nightcrawler had it worse. For starters, his monstrous (and permanent) look makes such a thing a lot more difficult than for those who look human, or those who can turn their inhuman look on and off (like Colossus and Iceman). Add to that being tortured by years at the Weapon X base. He's finally out, but in the X-Men he's Alone Among the Couples. In the end, he goes nuts.
- In the alternate timeline presented in Marvel Zombies, the death of his children at the hands of a zombiefied She-Hulk caused Reed Richards to suffer a mental breakdown that led to him deciding that the zombies represented the next stage of evolution. As a result, he not only deliberately infected the rest of the Fantastic Four with the virus, but once turned himself, he attempted to "spread the gospel" to other realities, only being defeated by his Ultimate counterpart using the magic of Doctor Doom.
Other
- Issue #5 of Hit-Girl shows Mindy having Hallucinations of Big Daddy, who advises her to go out and slaughter Ralph Genovese and the rest of his men in the horrifying ways that she'd been planning.
- Though the titular character of Johnny the Homicidal Maniac starts out by no means sane, he somehow manages to get worse over the course of the story, as his telling of the story gets steadily less believable, he gets talkier, less predictable and increasingly suicidal. It helps that he's being targeted by The Corruption (maybe), lives in a Mobile Maze, and regularly murders people.
- However, toward the end of the story, Johnny does kill himself and comes back (maybe), after which he stops hearing compelling voices and seems much saner. He does, however, still murder people.
- Teddy of Plutona slides from hero-worshipping the capes to wanting to become one to pursuing some pretty icky attempts to fulfill that dream.
- Primal Warrior Draco Azul: As a result of his initial lack of combat experience and rushed training, Ekchuah laments that four months of nonstop fighting have been eating away at Eric's sanity and caused him to become increasingly bloodthirsty and vicious in combat—lapsing into berserk states where he relentlessly mains his opponents.Ekchuah: You see, Eric and I have been at this nonstop. He's become more... unhinged over time. But this is something else. All of Draco Azul's past pilots have been strong-willed warriors with years of mental and physical training. Not only has Eric's training been rushed, but this war of ours might've been detaching him from the outside world. At this point, there's something else fueling his spirit. Something vengeful... primal even.
- Red Sonja: Queen of Plagues first shows Dark Annisia as a slave alongside Sonja, then as a high skilled military commander. As the plague's true nature is revealed she goes from Well-Intentioned Extremist down to bloodthirsty and indiscriminate murderer.
- Sandcastle: As time passes on the beach, Charles begins to develop Alzheimer's, which makes him a danger to himself and others, especially Amesan.
- Sonic the Hedgehog:
- In Sonic the Comic in the events leading up to the Sonic Adventure adaptation, Robotnik begins to lose it, first trying to destroy Mobius, and later gathering the Chaos Emeralds, the Freedom Fighters, and Chaos itself to his retreat so they would all die together.
- Dr. Eggman in Sonic the Hedgehog (Archie Comics) goes into a slow burn example after Sonic, Tails, Knuckles and Amy are able to rescue the captured citizens of Knothole, set them up in the nanite-built city of New Mobotropolis, and defeat the machine that defeated Sonic earlier. Things get worse when Knuckles-as-Enerjak destroys Eggman's city and sky fleet and, by the time issue 200 rolls around, Sonic delivers one last defeat that causes Eggman to flip out, tear his mustache apart and devolve into a blabbering mess.
- The Transformers (IDW):
- Whirl's mental state wasn't at its best at any point after his watchmaking business was destroyed to punish him for going against his assigned social caste, but it's shown to take a sharp dip in stability after he's thrown out of the Wreckers for attempting to Mercy Kill a comatose Springer. He only ends up on the Lost Light in the first place because Cyclonus stumbled on what may or may not have been Whirl attempting suicide atop a pile of mutilated Sweep corpses.
- Guzzle starts out a bit iffy in The Transformers: Last Stand of the Wreckers. By The Transformers: Sins of the Wreckers he's lost it so badly he needs to be kept in a cell when murder isn't required.
- Pharma started out compromising his morals a bit in order to keep his hospital running on a planet in the DJD's stomping grounds. Then he decides to stage a plague in order to get out of that compromise. Then he loses his hands when his role in the plague is discovered. By his return in "Remain in Light" he's a sadistic killer with a warped sense of humour who Word of God compares to Mark Hamill's Joker.
- Tyrest was once a respected mediator and the Chief Justice of Cybertron. Then he concluded that in his long-ago scientific experiments that created the process of cold construction, he'd given rise to a kind of Cybertronian that was "predisposed to sin". His resultant guilt complex leads to self-flagellatory drilling, which ends up causing brain damage, and the interference of a Mind Control gun only sends him further over the edge until he concludes that the only way to clear his conscience is to wipe out the constructed cold. When he shows up working for the Grand Architect, he's kind of recovered, at least.
- Played realistically with Fortress Maximus and Red Alert, whose trouble handling trauma and worsening paranoid delusions respectively are portrayed as serious issues that harm their lives and the people around them, with Fort Max having a traumatic breakdown and nearly killing several people and Red Alert attempting suicide.
