In one of the many endings of Catherine -and stop reading here if you hate spoilers- Vincent becomes corrupt and seeks a life in the netherworld with Catherine, eventually losing his human form. If that's not enough he becomes a powerful demonic overlord within a week.
Anyways, is it not possible that there are corrupt humans that live in the dark realm?
I am the black sheep "of course I'm crazy, but that doesn't mean I'm wrong."-Robert Anton Wilson