Spooky is right, some people have zero impulse control and zero forethought and do terrible things. That is not mental illness it’s poor decision making and thinking skills. Half these people are brain rotted and do not care about anything other than themselves.
Aristotle defined it as separate from insanity. Iirc he had three or four categories of general mental deficiency, insanity, stupidity, malevolence, and barbarity. I think this'd fall under barbarity, it's just kind of hard for modern people to identify since we don't see it nearly as much as people have historically