For me, it's people who are deplorable but don't try defending themselves. Murderers, rapists, and thieves who could hardly give a shit about most other people. Hell, corrupt politicians who are too busy trying to get power to care about morals or otherwise. I do find them even better if there's a likable quality to their personality, not anything redeeming. Like, an assassin who's casual about their job and chatting with the hero while he's trying to kill him.
That to me is more human than any kind of ambiguity. The fact people can be devoid of morals and principles while still being capable of enjoying another person's presence. Makes you feel like something is genuinely wrong there. But, in reality, that's just how some people are.