[Coda]

I don't think anyone does it "for no reason." People don't work that way. There's always some reason why they do things.

Some people might enjoy playing the villain (a recent-ish study showed that a large number of second playthroughs of games with moral systems play the evil route). At that point, it's not for no reason; it's because that's what the character would do. And that doesn't directly imply mental illness, because actors play sadistic villains all the time and can even enjoy doing so. Yes, someone might choose to play the villain that way BECAUSE of a mental illness, but it's not a prerequisite.