Says the history of the show. Joss consistently accomodates his mythology to his narrative demands.
F.ex., in Nightmares Vampire!Buffy helps her friends. And, before you argue that "it's a dream so it doesn't count", I want to elaborate that, again, I'm not talking about mythology; I talk about writing techniques. When Joss needs Vampire!Buffy cooperating with Scoobies, he makes her cooperative. But in Doppelgangland he needs a completely evil vamp!Willow - and she's completely evil.
Buffyverse mythology is so rich and so fuzzy that a writer can find a basis for practically every twist.
So basically nothing of the sort has happened before and you just think it might happen because Joss is terrible at continuity? Funny how even I, the guy who thinks Joss has totally lost it, don't think he is that bad at continuity. And anyway, if I agree with your premise, speculating becomes rather pointless as Buffy becoming a pink elephant and Xander becoming Superman would be be just as likely as your theory.
I agree that it's bizarre but we see in "What's My Line" that Dru the vampire is extremely and passionately upset over the loss and suffering that her family experienced at Angel's hands.
I always fast-forward that scene but I reckon Dru was just messing with Angel and pretended to be hurt by his actions.