I was thinking something like this but not as an opposition to vampires as not all vampires are evil...
Possibly have a bird-winged form lifestate (the angel) who is mainly aligned with good and have a darker (the demon, complete with tail) bat-winged form for evil-aligned sims? Then again, not all angels are good and not all demons are evil... so it would probably have to be dealt with through occult traits, not on a good/evil trait alignment.
It would be neat if they gained their motives from Good/Bad emotions depending on the type of sim. Like Angels feeding off of Good and Happy Emotions and Demons feeding off of Negative and Angry emotions. I mean, being around other sims who have those emotions equates to feeding, not like a Vampire compelling your sims for their plasma. I can be pretty confusing at times.
But... that would be bringing in religion to the game. Or at least a religion-aligned entity. They'll probably never add something like this due to that but there are modders who are working on adding additional lifestates, like mermaids. I do not know how far they've gotten on it, but if that is doable, then this should certainly be as well! I was watching to see their progress.