Murder Makes You Crazy

"Killing... after a while it infects you, and once it does you're never rid of it."
The Doctor in "The Doctor's Daughter."

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 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 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.

Examples:

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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 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.

     Comic Books  

     Film  

     Live-Action TV 
  • 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.

     Literature  

     Tabletop Games  
  • World of Darkness: Due to the way Morality system is designed, this trope is almost unavoidable. When you murder someone, your psyche cracks a bit and make it easier the next time you have/want to do it again. Do this often enough, and you turn into a Hollywood-style psycho, known as a Slasher.

     Video Games  
  • 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.
  • 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.
http://tvtropes.org/pmwiki/pmwiki.php/Main/MurderMakesYouCrazy