Really, the biggest hurdle you've got for yourself with this guy is that he can't speak mortal languages. It won't do him much good to understand us if all he'll ever be hearing is how monstrous he is and how much everyone wants him dead! I'm fine with the character's existence, but I think he needs to have some means of communication. And what that is is fine by me - he can talk, sign, inject his thoughts directly, what have you, so long as he can communicate with everyone else.
The other thing is, why would a character like this be working for someone as opposed to setting up their own group or, even better, staying out of mortal affairs? It's obvious why the Dark characters would want him on their side, but what can they offer him that would make him join up?
Aside from that though, I think it's a decent character and an interesting concept.
