Murder Makes You Crazy

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.

Examples of Murder Makes You Crazy include:

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, in the anime only. He's an ordinary school student until he write 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, this first target is skipped over, showing instead Ryuuk showing up after he's already killed numerous targets.

Comic Books

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.

Film

Evil Dead 2 - Ash, after being attacked by his possessed girlfriend Linda.

Ash: (Talking to mirror) I'm fine...I'm fine...
(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"?!

Literature

Used in 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.
Both Macbeth and his wife has this happen to them after their murder of Duncan.

Video Games

In Fahrenheit (2005 video game), 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.

Real Life

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.