Oh, that's interesting! I can't think of any examples of this I've encountered, but I do not think I would like it either.

but I think it also has to do with how that plotline resulted from his specific curse and tied into the world-building; it wasn't something thrown in because the writers had no idea what else to do, and there were definitely consequences. Or hmm, maybe it's because he was Angelus to begin with, so it wasn't the same as an ~always-heroic~ character getting turned evil.

Yeah, it's really well done in BtVS, but I think in general that's hard to pull off.