One Piece's Bloody Psychopath
When a tortured soul from a far-away future ends up in the hands of a bored god, he ends up being put into the body of a certain protagonist's sister.
Unfortunately for everyone involved, combining an almost suicidal maniac with a distinct lack of moral and a world where murder is common occurrence... Well, it doesn't really end well.
Thankfully, the bored god had at least the decency of giving a pretty useful gift to the soul. Less thankfully, it does little in helping the serious mental issues the soul possesses.