"Killing... after a while it infects you, and once it does you're never rid of it."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 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 name of this trope is from Love Makes You Crazy, so contrast this. Also related to It Gets Easier, and possibly Shell-Shocked Veteran. The difference between this and a normal Face–Heel Turn, is that it 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.
— The Doctor in "The Doctor's Daughter."
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Anime and Manga
- 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. 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 be pinpointed exactly to the point where she killed her parents. In fact, in a version of the past where she didn't do this, she recovered and became a hopeful and optimistic girl.
- 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.
- 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, Superman appoints himself Judge, Jury, and Executioner and uses kryptonite (keeping himself in a Kryptonite-Proof Suit of course) to kill them. Superman was later so emotionally disturbed about this that he developed a Split Personality that took the form of an extreme Nineties Anti-Hero.
- 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.
- 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"?!
- 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.
- 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 has 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.
- 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.
- Falkland’s 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.
- There may be some Truth in Television, in the cases of crimes of passion. More research would be needed though.
- There is an actual condition called Post Traumatic Stress Disorder, which happens mainly to ex-soldiers, and is pretty much the Real Life basis for the Shell-Shocked Veteran. Killing people isn't the only way it can come about though.