Normally, in Real Life (although there may be exceptions), a person is already crazy and/or violent before they contemplate murder. In television, however, circumstances will cause you to commit murder, and then you keep killing. No, this trope is not about murder where you have to keep killing in order to "hide the evidence". Instead, it's about murder actually warping the mind, either by guilt or as the process of rationalizing the murder sets in.
Usually, this involves some lead-up event where they are sane but have decided to kill from being put into a situation where, unless they are an Actual Pacifist with a knack for taking third options, they will practically be forced to murder because of a no-win situation. After doing the actual murder, however, the act makes them evil, or in this trope, insane.
The difference between this and a normal FaceHeel Turn, is that this usually follows excessive amounts of guilt and gloom, and results not in evil, but rather madness. Still, it can probably be viewed as a sister trope. Likewise for Slowly Slipping Into Evil, since it isn't evil you slip into, but rather madness.
This is a trope specifically pertaining to murder (and insanity). Expect spoilers.
- In Cage of Eden, this is what breaks the ace, Kouhei, combined with a series of misunderstandings and some light brain damage.
- Light Yagami of Death Note. He's an ordinary school student until he writes the first name in the titular Artifact of Doom. After that, he develops delusions of grandeur and wants to cleanse the world of evil (leaving himself as the only evil person, as Ryuuk points out). In the manga, the reader first sees how Light has already killed numerous targets before flashing back to his first kill for maximum shock value and to show how far he has already fallen in such a short time.
- Yuno Gasai of Future Diary is probably the Trope Codifier. Not only is she pretty much the poster girl for Yandere, but her entire issues can be pinpointed exactly to the point where she lost it, snapped on her parents for their abusive and controlling behavior, and killed them. In fact, in a version of the past where she didn't do this, she recovered and became a hopeful and optimistic girl.
- In A Cruel God Reigns, Jeremy suffers multiple nervous break-downs, hallucinations, and becomes a drug-addicted prostitute as an after effect of killing his step-father and his mother (although his mother was an accident).
- Puella Magi Madoka Magica: In the manga (but not the anime), Sayaka's final straw is her murder of two random guys who, all things considered, didn't really deserve being killed in cold blood. By her own admission, she is such a fool.
- Reiner Braun from Attack on Titan is driven insane by the guilt of having committed mass murder, developing a Double Consciousness in order to keep going. In particular, the murder of Marco Bott to protect their secret causes him to completely break down, immediately lapsing into a Dissociative state and pleading to know why their friend is being eaten.
- In the Post-Crisis universe, Superman is forced to kill three Kryptonian criminals that wiped out an alternate version of Earth. Since there are no longer any officials to sentence them on their world, no legal precedent that would let them be imprisoned in his world, and they have already vowed to find some way to regain their powers and go after him in revenge, Superman appoints himself Judge, Jury, and Executioner and uses kryptonite (immune to it himself as this was kryptonite from a different universe) to kill them. Superman was later so emotionally disturbed about this that he developed a Split Personality that took the form of an extreme '90s Anti-Hero, temporarily abandoning Earth until he was helped to see that he only sinned in the cause of justice under extreme circumstances.
- More than one writer has cited this trope as a reason behind Batman's Technical Pacifist stance. He fears he is so close to the ragged edge of sanity already that if he starts killing anyone, he will not be able to stop. In one alternate universe shown in Countdown to Final Crisis, he kills The Joker and then decides he might as well kill every other supervillain — and succeeds.
- In Batman Vampire, after being vampirized by Dracula, Batman reflects at one point that should he succumb to his bloodlust even once, he'll become a monster just like the Count. Ultimately downplayed; draining the Joker got the ball rolling, but it was the months he spent in a false death rotting that sent Batman over the edge.
- Sweeney Todd: The Demon Barber of Fleet Street both plays this trope straight and subverts it; although the trigger for Sweeney's crowning moment of crazy is the Judge escaping before "Epiphany", Sweeney has already been forced to kill Pirelli, an Asshole Victim who tried to blackmail him, but was thoroughly unrelated to his initial reason for revenge. His descent from a single-minded vigilante out for revenge to a raging psychopath willing to kill anybody arguably comes about as a result of the earlier kill, in that he's already begun killing men who aren't responsible for ruining his life.
- Evil Dead 2: Ash, after being attacked by his possessed girlfriend Linda.
Ash: (Talking to mirror) I'm fine...I'm fine...Mirror Ash: I don't think so. We just cut up our girlfriend with a chainsaw. Does that sound "fine"?!
- Friday the 13th: After killing Jason in The Final Chapter, Tommy Jarvis spends years in and out of various mental institutions due to the trauma.
- In Doctor Who, The Doctor tries to advise his daughter grown in a military cloning device as a soldier that violence is not the answer. There have been more than a few times where people go What the Hell, Hero? to the Doctor, so he's definitely speaking from experience. After the Time War he knows, really knows, how killing can mess up the killer.
- Buffy the Vampire Slayer's Faith starts to go off the deep end after she kills a man who she thought was a vampire. Angel later tries to help her come around by establishing common ground. He calls the ability to kill without remorse the ultimate freedom, and a freedom that will drive anyone mad. And of course he's taking from experience.
- Ed Nygma in Gotham was initially decidedly eccentric and even a little creepy at times, but nothing worse than that. After a confrontation with his crush's abusive boyfriend ended in Ed stabbing the guy, he began showing signs of Sanity Slippage.
- As of season 5 of Breaking Bad, creator Vince Gilligan has stated, "The new Walt lives in a power vacuum created by the death of Gus Fring." Given his actions before the death of Gus Fring, this is really saying something.
- Used in The Dresden Files and the reason for the First Rule — magic is an expression of will given form, so using it to kill someone is particularly warping and even addictive. Non-magical killing doesn't cause this, though Harry confesses in one book that he's haunted by having to execute two people, and fears that this makes him a monster as well.
- Both Macbeth and his wife have this happen to them after their murder of Duncan.
- Agatha Christie's Hercule Poirot discusses this trope a few times. He notes that once someone gets over the initial mental hurdle of taking someone's life for the first time, to them murder becomes an acceptable solution to any number of life problems. This becomes particularly explicit in the final Poirot novel, Curtain; faced with a man who has manipulated others into killing for him without ever actually committing a single crime himself, Poirot chooses to kill the man himself, and then it is strongly implied that Poirot let himself die of his current heart condition so that he wouldn't succumb to this fate and start believing that he had the right to kill those he deemed it necessary to eliminate.
- In the Harry Potter series, committing murder tears the soul. The soul can heal from this, provided both pieces are left next to each other. However, Dark wizards have been known to exploit this soul-tearing by using it to make Horcruxes.
- Falklands life goes decidedly downhill after he murders Tyrrel in Caleb Williams. He goes from being the most popular squire in his county to a man who avoids even his servants, and goes wandering at night in stormy weather.
- In Fahrenheit, Lucas thinks that he is going crazy after (unwillingly) murdering, a man but it is later revealed that his mind has been manipulated and damaged by one of the villains all along.
- Oracle of Tao has Ambrosia, who is more or less a little crazy already (being a Mood-Swinger Sugar-and-Ice Personality with a Literal Split Personality), but in a Bad Ending, she goes noticeably over the edge after killing an angel. She starts talking about "balancing the scales" (which, since she created the universe from a Dream Apocalypse, means basically destroying everything), and goes on a homicidal rampage, even killing her own party.
- This is part of the sanity-tracking game mechanics of Lone Survivor. Killing mutants might be the most expedient way of getting past them, but it has a negative effect on your score, and the more you kill, the worse it gets.
- Every time you kill someone in Yandere Simulator, you lose Sanity. Lose too much Sanity without an effort to regain it, and people won't want to deal with you — least of all Senpai. YandereDev has stated, however, that Yandere-chan is already insane, and that the Sanity Meter just shows how well she is hiding that fact.
- Higurashi: When They Cry: Keiichi and Rena, after killing Teppei and Rina, respectively.
- Happens to Clover in Nine Hours, Nine Persons, Nine Doors during the "axe ending". She starts by wanting to kill Santa and Seven, believing them to be responsible for the death of her brother. June was trying to defend them, so she killed her too. After 3 murders, she snapped completely and killed Junpei for no particular reason.