One of my players is vengeance paladin. I had him choose a favored enemy. He chose “The evil gods and their devout followers.” I love that, cause it fits his character perfectly. No way would I restrict his special abilities to just those people, though, except, perhaps, the vow of enmity. Even then, I’d probably be very loose with it, as in he can’t target someone whom he knows for certain follows a good or neutral god.