What someone says when they have done something for the purpose of maliciously disrupting another person(s) motive, intentions, life, job, etc, and the victim asks that someone why they did it.

A response to a question of the practicality of something which is clearly overkill, it implies that the one who uses the phrase to answer the question of 'why' knows full well that it is overkill, and overkilling for overkill's sake was their intent.

(The examples probably describe it better)

The correct response to a smart arsed sarcastic question, usually from your partner.

BECAUSE FUCK YOU, THAT's WHY

Because Fuck You, That's Why. A simple and clean reason to do anything without any reasons provided... or better yet, the reason is "Fuck you". This phrase is used widely in memes.

Sometimes a simple FUCK YOU isn't enough. In the most violent fits of verbal rage things like morals, ideals, and at times even sexual preference get thrown aside to make room for the worst insult possible.

A man telling another man that he will fuck him faggot is quite possibly the end all to insults. Just the thought of being fucked up your asshole until you start liking it is enough to make most men tuck their dicks between their legs and abandon the argument or beef.

Can also be used like:

He fucked me faggot, I got fucked faggot, did you hear about the guy Jim fucked? He's faggot now.

A polite way of saying, "Fuck you!"