This is a great argument! I'm adding it to my meta hall of fame. :)

I think this also gets at a phenomenon that bothers me in some fic- the assumption that Buffy hasn't killed Spike because she's secretly soooo attracted to him (cheekbones of doom y'know). I do believe there is chemistry (though I think it's completely sublimated into disgust until Intervention), but if love won't stop her from killing Angel, I don't easily accept that attraction would keep her from killing an (icky!) vampire like Spike. Buffy's morals and keeping her word being the reason, that I can believe.