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->''"Killing... after a while, it infects you, and once it does, you're never rid of it."''
-->-- '''The Doctor''', ''Series/DoctorWho'', "[[Recap/DoctorWhoS30E6TheDoctorsDaughter The Doctor's Daughter]]"

Normally, in RealLife (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 [[NeverOneMurder murder where you have to keep killing in order to "hide the evidence"]]. Instead, it's about murder ''actually warping the mind'', most often because of the [[MyGodWhatHaveIDone guilt]], shame, and horror that sets in as one processes [[NeverMyFault the murder they just committed]]. However, there are also times when a character's mind breaks because they've come to the realization that [[EvilFeelsGood they quite liked killing]].

Usually, this involves some lead-up event where they are sane but [[GainingTheWillToKill have decided to kill]] from being put into a situation where, unless they are an ActualPacifist with a knack for [[TakeAThirdOption taking third options]], they will practically be forced to murder [[SadisticChoice because of a no-win situation]]. After doing the actual murder, however, the act drives them to insanity.

The difference between this and a normal FaceHeelTurn is that this usually follows [[BreakTheCutie 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 SlowlySlippingIntoEvil, since it isn't evil you slip into, but rather madness.

Compare and contrast LoveMakesYouCrazy. Related to ItGetsEasier, SanitySlippage, and possibly ShellShockedVeteran. Frequently is accompanied by ScrubbingOffTheTrauma and can lead to one becoming AxCrazy.

!!As this is a {{Death Trope|s}}, [[Administrivia/SpoilersOff unmarked spoilers abound]]. [[Administrivia/YouHaveBeenWarned Beware]].
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!!Examples:
[[foldercontrol]]

[[folder:Anime & Manga]]
* In ''Manga/CageOfEden'', this is what [[BrokenAce breaks the ace]], [[spoiler:Kouhei]], combined with a series of misunderstandings and some light brain damage.
* [[Characters/DeathNoteLightYagami Light Yagami]] of ''Manga/DeathNote''. He's an [[OrdinaryHighSchoolStudent ordinary school student]] until he writes the first name in the titular ArtifactOfDoom. After that, he develops [[AGodAmI delusions of grandeur]] and wants to cleanse the world of evil (leaving himself as the only evil person, as Ryuuk points out; Light is already too into his delusions of being a god of justice to understand that). 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.
** Later Light pulls a MemoryGambit, forgetting that he's Kira and reverting to his personality before receiving the Death Note. This Light is brilliant but innocent, and completely horrified by the possibility that he could be a killer. Thus highlighting again just how ''quickly'' he snapped after killing for the first time.
* Yuno Gasai of ''Manga/FutureDiary'' is probably the TropeCodifier. 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, [[spoiler: 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 ''Manga/ACruelGodReigns'', Jeremy suffers multiple [[spoiler: 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).
* ''Anime/PuellaMagiMadokaMagica'': In the manga ([[DependingOnTheWriter but not the anime]]), [[spoiler:Sayaka]]'s final straw is her murder of two random misogynistic guys who, all things considered, didn't really deserve being killed in cold blood. [[AndThenJohnWasAZombie By her own admission, she is such a fool]].
* [[spoiler:Reiner Braun]] from ''Manga/AttackOnTitan'' is driven insane by the guilt of having committed mass murder, developing a DoubleConsciousness in order to keep going. In particular, the murder of [[spoiler:Marco Bott]] [[HeKnowsTooMuch 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 ''Manga/ShiNiAruki'', [[spoiler:Sou Aoya]] killed [[spoiler:his adopted sister Ai's AbusiveParents]], then becae convinced that he's a monster who had to drink the blood of "pure" people to purify himself. Even Tokiko, who has a disturbingly calm attitude to the death that seems to follow you, is appalled at his mindset.
[[/folder]]

[[folder:Comic Books]]
* In the Post-Crisis universe, ''ComicBook/{{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 JudgeJuryAndExecutioner 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 SplitPersonality which took the form of an extreme NinetiesAntiHero, 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 Franchise/{{Batman}}'s TechnicalPacifist 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 ''Comicbook/CountdownToFinalCrisis'', he kills ComicBook/TheJoker and then decides he might as well kill ''every other supervillain'' -- and succeeds.
** In ''ComicBook/BatmanVampire'', after being vampirized by {{Dracula}}, Batman reflects at one point that should he succumb to his [[HorrorHunger 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 [[AndIMustScream in a false death rotting]] that sent Batman over the edge.
[[/folder]]

[[folder:Films -- Live-Action]]
* ''Film/SweeneyToddTheDemonBarberOfFleetStreet'' 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 AssholeVictim [[BlackmailBackfire who tried to blackmail him]], but was thoroughly unrelated to his initial reason for revenge. His descent from [[TheDeterminator a single-minded]] [[RoaringRampageOfRevenge vigilante out for revenge]] to a [[AxCrazy raging psychopath]] [[TheUnfettered 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.
* ''Film/EvilDead2'': Ash, [[WhatHappenedToMommy after being attacked by his possessed girlfriend Linda]].
-->'''Ash:''' ''(Talking to mirror)'' I'm fine... I'm fine...
-->(''[[MirrorMonster His reflection lunges out of the mirror at him]].'')
-->'''Mirror Ash:''' I don't think so. We just cut up our girlfriend with a chainsaw. Does that sound "fine"?!
* ''Franchise/FridayThe13th'': After killing Jason in ''[[Film/FridayThe13thTheFinalChapter The Final Chapter]]'', Tommy Jarvis spends ''years'' in and out of various mental institutions due to the trauma.
[[/folder]]

[[folder:Literature]]

* ''Literature/CrimeAndPunishment'': Raskolnikov grows increasingly deranged after committing murder. It is only when he has his HeelRealization and decides to confess that he calms down, and even then it takes until the DistantFinale for him to finally find peace.
* Used in ''Literature/TheDresdenFiles'' 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.
* Creator/AgathaChristie's Literature/HerculePoirot discusses this trope a few times. He notes that [[GainingTheWillToKill once someone gets over the initial mental hurdle of taking someone's life for the first time]], [[ItGetsEasier to them]] [[MurderIsTheBestSolution murder becomes an acceptable solution to any number of life problems]]. This becomes particularly explicit in the final Poirot novel, ''Curtain''; [[spoiler: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 ''Literature/HarryPotter'' 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 [[SoulJar Horcruxes]].
* Falkland’s life goes decidedly downhill after he murders Tyrrel in ''Literature/CalebWilliams''. 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.
* Hollyleaf in ''Literature/WarriorCats'' goes mentally downhill after killing Ashfur to preserve the secret of her true parentage, though part of this is her feelings of betrayal and being born a crime stemming from that parentage reveal. By the end of her breakdown, she ends up revealing the secret to everyone despite killing to keep it.

[[/folder]]

[[folder:Live-Action TV]]
* In ''Series/DoctorWho'', The Doctor tries to advise his daughter [[spoiler: 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 WhatTheHellHero 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.
* ''Series/BuffyTheVampireSlayer'': [[spoiler:Faith]] starts to go off the deep end after she [[spoiler: 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 [[AxCrazy he's talking from experience]].
* Ed Nygma in ''Series/{{Gotham}}'' was initially decidedly eccentric and even a little creepy at times, but nothing worse than that. After a confrontation with his crush's [[DomesticAbuser abusive boyfriend]] ended in Ed stabbing the guy, he began showing signs of SanitySlippage.
* As of season 5 of ''Series/BreakingBad'', creator Vince Gilligan has stated, "The new Walt lives in a power vacuum created by the death of [[spoiler:Gus Fring]]." Given his actions ''before'' the death of [[spoiler:Gus Fring]], this is really saying something.
[[/folder]]

[[folder:Tabletop Games]]
* ''TabletopGame/WorldOfDarkness'': Due to the way [[KarmaMeter 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 [[TabletopGame/HunterTheVigil a Slasher]].
[[/folder]]

[[folder:Theatre]]
* Both Theatre/{{Macbeth}} and [[LadyMacbeth his wife]] have this happen to them after their murder of Duncan.
[[/folder]]

[[folder:Video Games]]
* In ''VideoGame/{{Fahrenheit}}'', Lucas ''thinks'' that he is going crazy after (unwillingly) murdering, a man but it is later revealed that [[spoiler:his mind has been manipulated and damaged by one of the villains all along]].
* ''VideoGame/OracleOfTao'' has Ambrosia, who is more or less a little crazy already (being a MoodSwinger SugarAndIcePersonality with a LiteralSplitPersonality), but in a Bad Ending, she goes noticeably over the edge after killing [[IncorruptiblePurePureness an angel]]. She starts talking about "balancing the scales" (which, since she [[spoiler:created the universe from a DreamApocalypse]], means basically destroying everything), and goes on a homicidal rampage, even killing her own party.
* When playing as Hibiki in ''VideoGame/TheLastBlade 2'', repeatedly using {{Finishing Move}}s will eventually result in her going AxCrazy. The same goes for her stint as a GuestFighter in ''VideoGame/SamuraiShodown (2019)''.
* This is part of the sanity-tracking game mechanics of ''VideoGame/LoneSurvivor''. 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 ''VideoGame/YandereSimulator'', 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. This only partially fits the trope, since by WordOfGod, Yandere-chan was ''already'' insane. The 'Sanity' meter just tracks how well she can keep on her MaskOfSanity.
[[/folder]]

[[folder:Visual Novels]]
* ''VisualNovel/HigurashiWhenTheyCry'': [[spoiler:Keiichi and Rena, after killing Teppei and Rina, respectively. Shion's first kill was actually an accident, since she wanted to interrogate Oryou about Satoshi. Afterwards, she realizes there's no turning back now, which makes her grow increasingly paranoid and unstable. Ironically, she might have convinced Shion had she survived, because Mion admitted that she herself nearly killed her grandmother in anger after Satoshi disappeared but Oryou managed to convince her that the Sonozakis were innocent.]]
* Happens to [[spoiler:Clover]] in ''VisualNovel/NineHoursNinePersonsNineDoors'' during the "axe ending". [[spoiler: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.]]
[[/folder]]

[[folder:Web Original]]
* ''Website/{{Springhole}}'': Brought up in the respective articles on [[https://www.springhole.net/writing/writing-mentally-ill-and-insane-characters.htm mental illness]] and [[https://www.springhole.net/writing/things-about-death-dying-and-murder-writers-need-to-know.htm murder]] to point out the trope is nonsense. Assuming someone wasn’t already prone to committing violence without remorse, a person who lashed out and actually killed someone would be far more likely to end up traumatized and [[MyGodWhatHaveIDone wracked with guilt]].
[[/folder]]
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