- In Amazing Fantasy, Years of constant defeats turned the already self-centered Mysterio into an individual obsessed with being seen as something more than the guy who Spider-Man beats, resulting in him committing much darker crimes on in both Izuku's and Peter's universes.
- Jack O'Lantern, the Arch-Enemy of Ultimate Spider-Woman, commits increasingly violent and destructive crimes as the series progresses. He goes from robbing banks to taking people hostage to going on killing sprees to orchestrating a bloody city-wide Mob War. Even in jail, we see him fantasizing about carving up human heads the way people do Halloween pumpkins. What's unusual is that Jack always had these sadistic urges-the Sanity Slippage comes from the escalating nature of his crimes.
- A Crown of Stars: Jinnai, this story's Big Bad, became much more paranoid (and a big time megalomaniac) since he overthrew former Big Bad Winthrop because he realizes that the same thing could happen again; only this time with his head on the chopping black.
- In the Undertale fanfic Alphys Hoek, this gradually happens to Alphys over the course of each chapter, partially as a result of having to deal with the game's fanbase 24/7 and also partially as a result of the unspeakably humiliating things that Mettaton forces her to do on public TV where everyone in America can see her; by the end of the story, she's a literal genderbent Ren Höek who's completely hell-bent on twisting Mettaton's neck in his sleep.
- Hetalia: Axis Powers: Austria/Roderich Edelstein in the 1983: Doomsday Stories is portrayed for the most part as undergoing through this. His obsession in finding out whether Hungary survived Doomsday grows increasingly desperate (and irrepressible) over time to the point that others eventually start wondering if he's gone delusional from grief. He doesn't. It's also mentioned to have happened as well with Romano (hijacked by The Mafia as his brother's successor) and Poland (having his personality shattered).
- Thousand Shinji: As the story progresses, the corrupting influence of the Chaos Gods upon Shinji, Asuka and Rei increases, and they became more psychotic and unstable. Shinji and Asuka become increasingly violent, the latter descending into berserker madness during two months Shinji was missing, killing criminals every night to lessen her pain and loss, and Rei turns their apartment into a garbage-filled shrine to decay.
- Calvin goes a little crazy trying to watch a pug in Calvin and Hobbes: The Series.
- Jack in Part 1 of Cave Story vs. I.M. Meen and for good reason. After being trapped inside an insane pedophile's sick mockery of a yuri pairing fanfic between his girlfriend and his girlfriend's best friend, being forced to work a part-time shift at a restaurant called Diarrhea Bell, being forced to watch the two girls 2-Girls-1-Cup each other right in front of his eyes, and being tickle-tortured and thrown in jail, Jack lets loose with this underrated gem of a scene, cementing this story's status as one of the secretly best fanfictions of all time:"I am Jack. Woe is me for I am wrongfully imprisoned. I am Jack! Sad and unhappy am I without friends. BOO HOO!! BOO HOO HOO HOO!!! (blows snot into handkerchief and smears it all over the floor)"
"Emptiness wells up in my tortured soul. I'm going to...GOING TO...(vomits all over the floor)"
"I am Sue; I also am indeed BLEAUGH! (dumps mixture of random condiments and gross stuff on the floor)"
"I'm just Toroko but I'm gonna barf too, and the MUSTARD, MUSTARD BARF MUSTARD BARF!!! (dumps ketchup and mustard all over the floor)"
"(soaks hands with the combined mixture) BEHOLD the GOOEY SLOP of our SORROW! (smears it all over his face) IT IS...SLOBBER...DAMMERÜNG! THE TWILIGHT OF THE CONDIMENTS!!! (removes hands from face and screams to the heavens, palms facing skyward) WEEP FOR THEM, WEEP FOR THEIR SORROW-HO-HOHO-HOHH!!!"
"Yes, Jack, my delicate psyche has come horribly unglued!"
- In the vein of the Cutie Mark Failure Insanity Syndrome described above in the Western Animation section, The Audience by Reality Check states that Trixie herself is diving into one as her special talent isn't magic like most unicorns, but rather stage magic, illusions and theatrical personalities. The constant heckling, jeers and challenges towards her achievements that she was receiving were making her the Jerkass that she was portrayed as in canon.
- Total Drama Stranded: Briony gets a major one very quickly when Calvin begins relentlessly bullying her.
- From the sister story, Total Drama Switcheroo, Missy gets an even bigger one.
- Queen of All Oni: Her encounter with Lung leaves Jade with some severe PTSD, which only worsens as time goes on. It finally culminates with her confrontation with Tohru in the Vault of Endless Night, which convinces her the heroes are all out to kill or seal her, at which point she snaps completely.
- The final story of Calvin and Hobbes: The Series has two separate plotlines: one concerning the return of Thunderstorm and another concerning the Slender Man showing up. When the stories intersect, Thunderstorm starts displaying signs of this.
- Nav starts showing signs of this in Diaries of a Madman, as he becomes far more reckless and depraved over the course of the story.
- Spike goes through one throughout Zenith, which stems from his Undying Loyalty to Twilight.
- Loved and Lost: Prince Jewelius was hardly ever sane to begin with, but after Princess Luna thwarts his attempt to hang Celestia, he gradually undoes all the hard work of his manipulations by losing his composure, responding angrily to subsequent setbacks in his plans, putting forth less effort into restraining his Berserk Button regarding the adoration directed towards Celestia and Cadance, and reacting petulantly to dissension/disagreements from others.
- Memories has Serenity. When we first see her, she's like Pinkie Pie, a happy, cheerful, harmless, fun-loving, chaotic, Cloudcuckoolander of a prankster with a soft spot for Discord. Then she dies and comes back to life. Cut to the epilogue and sequel and she's giving out Slasher Smiles, maniacal laughter, telling Discord that she wants to overthrow his mother and rule in her place, and finally saying that together they'll kill Celestia.
- In Being Dead Ain't Easy, Seto Kaiba slowly loses his mind as the story progresses for a variety of reasons, including guilt over Joey's death, obsessively trying to find a way to save Joey, lack of adequate food and sleep, KaibaCorp not doing well, worry for his own life and Mokuba's, and synchronization with the dying Joey.
- The Rise of Darth Vulcan: The Alicorn Amulet should be doing this to Ted/Darth Vulcan, and the fact that it doesn't appear to be is worrisome for the heroes. Comments by Word of God, and in-universe by the Princesses regarding Ted's "True Self" being appalled at his actions, suggest that he was suffering from this even before he put on the Amulet, from years of convincingly pretending to be a cruel bully in order to avoid bullying himself. As such, the Amulet can't drive him insane, because he was never quite sane to begin with.
- This happens to some characters in The Infinite Loops due to the strain, frustration, and boredom of living through the same events many many times. Sakura is the poster child for this sort of thing, having gone off the deep end and lost all moral restraint due to having come to believe that nothing she does in the loops matters since the loop will just reset.
- Occurs in the Legend of Korra fanfic Book 5: Legends. Temujis plan in the final chapters seems half-thought out at best, and gets worse the longer they go without fulfilling it, culminating in a Villainous Breakdown worthy of Azula's.
- The ABC's of Fallout: Equestria, a collection of short stories, has Irradiate, witch tells the story of a mare who becomes a ghoul and slowly goes feral.
- In Ave Sonata, this is apparently why Sonata Dusk is The Ditz.
- In The Story to End All Stories, the villain has gradually descended into madness to the point that he is unrecognizable.
- Estee's Triptych Continuum has one in Permanent Record, wherein a poor guard is driven to utter and absolute madness over time by the antics of the Cutie Mark Crusaders, eventually culminating in their utter certainty that everything which goes wrong is their fault. To be fair, though... usually it is.
- Le Papillon Rising takes Adrien Agreste from Miraculous Ladybug, and slowly twists him into a psychotic, obsessive, slightly sociopathic Yandere through use of the Butterfly Miraculous. It's worth noting that this is an AU where Adrien is the main villain.
- Transformers Animated: Cybertronian Genesis: One of the main focuses of the narrative for both the original story and its sequel is Megatron's slow descent into madness as his obsession with defeating Optimus Prime grows. Eventually, it becomes so bad that Starscream calls him out on it. He only manages to snap out of it when Gigatron gets involved.
- Miranda Lawson under goes this in It Got Loose and Killed All Our Guys: The Project Lazarus Story, brought on by a mixture of being asked to bring someone back from the dead, the Illusive Man randomly asking for her to add modifications to Shepardnote , dealing with her (rapidly dying) co-workers, and the sheer isolation. She eventually declares Dead Shepard her best, and only, friend, and feels betrayed when he comes back to life.
- Reaper was already Ax-Crazy in Overwatch canon. In Break My Heart, Break Your Heart, he gets worse. Way, way worse.
- This Bites!: During the Enies Lobby arc, Sengoku combines this with Unstoppable Rage. Every piece of bad news that comes his way drives him closer and closer to the edge, and by the time he's finally on his way to the Judicial Island himself, he's basically in full-out Villainous Breakdown mode.
- Twilight and Pinkie Pie go though this in Bride of Discord, after Discord takes Fluttershy away, do to both losing a friend and in Twilight's case, working herself to the bone and refusing to rest, while trying to find any possible way of getting her back.
- Code Geass Megiddo: Suzaku has been on a gradual descent to insanity after he murdered his father to save Lelouch and Nunnally's lives, but after Euphemia's death and the events that transpired as a result of that (including learning that his best friend was his Arch-Enemy), that descent accelerated and never stopped. By the time the narrative truly starts, Suzaku's Mask of Sanity is barely holding on and begins to crack when Lelouch is kidnapped by the Black Knights, cementing his inevitably return as Zero, and full-on breaks when they finally confront each other at Pearl Harbor. At this point, several other characters have become aware of how insane Suzaku truly is, and believe that if Lelouch still cares for Suzaku, on any level, killing him would be merciful.
- Team Rocket Roots: After her favorite agent, Miya, went mysteriously missing while on a mission, Team Rocket's Madame Boss Gianna began slipping.
- Rocket To Insanity is a fanfic where Cupcakes is a nightmare of Rainbow Dash's. She keeps on having it over and over again. It gets to the point where Rainbow Dash withdraws from her life and distances herself from everyone else. When Pinkie Pie tries to cheer up Rainbow with a cupcake, Rainbow snaps and kills Pinkie in what-she-thinks is self-defense.
- Satsuki undergoes this in chapter 5 of Come find me, again., due to depression over the loss of her sister. After noticing some changes, her parents make her come stay with them (or she'll be put in the hospital) and, when she's brought home, she gets worse, as she starts hallucinating, dancing on the widow's walk railing (almost falling off), and even broken glass (without shoes). Afterwards, she's hospitalized, where she starts to improve.
- In Infinity Crisis:
- Daisy notes that the Red Skull must have gone insane after being trapped on that planet guarding the Soul Stone, but Steve simply observes that it would have been a very short trip.
- In Of Kryptonians and Queens, Merlin states that whatever dark arts Morgana used to extend her lifespan over centuries made her more dangerous and twisted than before.
- In Gamma Relations, Bruce immediately tells that Sterns' transformation into the Leader also drove him insane.
- Starting from about one-fourth into The Very Secret Diary, Ginny begins blacking out, sleepwalking, and behaving unusually enough that her family starts to worry. This is thanks to Tom Riddle possessing her without her knowledge. Ginny, not realizing Tom's responsible, confides in him, making it that much easier for him to Gaslight her and make her question her own sanity. Once she finally realizes what's really going on, things get worse. Much, much worse.
- Supergirl (2015) fanfic my youth is yours: Flashbacks show how Lex went from Lena's cool big brother who was very supportive of her relationship with Kara to a violent human-supremacist willing to do anything to take down Superman.
- Fireside Tale: Anna grieved over Elsa for the rest of her life. She was considered "mad". Even when Arendelle became uninhabitable and everyone else left, Anna stayed.
- Kingdom of Isolation: Anna's brutal death takes a toll on Elsa. She shuts herself from everyone and creates a "perfect world" of snow people where Anna is alive and it's her coronation every day. Her parents and Anna are all alive while Hans is a pile of slush in the prison. In the real world, Arendelle is in turmoil because there's a raging blizzard.
- Platinum from A Brief History of Equestria suffered from this during the Winter War due to her plans for Unicornia falling apart. Clover had to lock her up in her chambers for her own good and take over the war effort. She broke out in time for the events leading to the Warming, and recovered after the Windigos were defeated.
- Kara of Rokyn: According to Nasthalthia's story, her uncle Lex Luthor used to be a hopeful, awkward, brilliant kid before running into Superboy. Lex initially befriended Superboy, but then he started blaming him for everything that went wrong in his life. At the beginning Lex merely wanted to show Superboy off, but as his obsession grew Lex gradually became madder, more irrational and more twisted, and quite soon he was plotting ways to murder his ex-buddy. When the proper story begins, Luthor has lost any redeeming qualities and is nothing but a ruthless, cold psychopath overwhelmed by inexhaustible hatred.
- In Dragon Ball Z Abridged, Piccolo and Vegeta suffer this when they spend time in the Hyperbolic Time Chamber by themselves. Piccolo starts hallucinating Cell after spending some time in there and decides that's enough training. Vegeta's the strange one as he resorts to a broomstick and a volleyball made up to look like Nappa after three days. In real time. Mr. Popo changed the time dilation in there, most likely in revenge.
- The Raven's Plan:
- Stannis' time in the Black Cells has clearly damaged his psyche. By the time Robert and Renly visit him, he's seething in rage at the very sight of them, demanding to understand why it is that he's always been The Unfavorite out of the three of them and wondering what Renly could have possibly done to earn Brienne's Undying Loyalty, and when he learns about Jon Snow's true identity, he starts laughing at them before proclaiming himself to the "one true king of Westeros, Jon Targaryen".
- Lysa was hardly the picture of good mental health before the Remembering, but after? She's madder than the Mad King and a threat to herself as much as she is to everyone else. It's so bad that Yohn Royce currently has her drugged to the gills with milk of poppy for everyone's sake.
- Much like Lysa, Cersei wasn't all there to begin with, but after she's thrown into the Black Cells she starts losing her mind. Not helping the matter is Robert, who is deliberately tormenting her as payback for their toxic marriage.
- Fire!: Firebrand was never the most stable villain, but he endeavoured to be a cold, cunning schemer. When his father dies, though, Firebrand completely loses any semblance of sanity and develops a "Kill them all" mindset.
- In Among Us In Character, Drama slowly loses it after she continuously fails to catch the killer, and dreams of her dead crewmates each night. It's later revealed this was the result of an alien parasite that infected her, which was slowly breaking her mind to take control of her. It succeeds.
Sanity Slippage / Fan Works