Isn't it kind of sinister, though, when a multi-genocidal serial rapist puppy-kicking total bastard is able to, you know, function normally and generally come across as an affable, polite individual? It's not realistic, but it's done, as you note, even in Hollywood. There's a reason for that: the juxtaposition is really effective.

That doesn't mean, of course, that everybody can do it or do it well. I have experienced truly awful 'evil' characters and find the female ones in particular tend to talk like dominatrixes. It comes across, to me, as more comically naff rather than threatening.