What Deird said - I think Buffy's view of Spike is fairly set by the end of Primeval when she rolls her eyes at his attempt to 'fix' his betrayal. She expects him to continue trying to be evil incompetently, because that's implicitly allowed as part of their deal. He wouldn't actually break his end until she actually felt threatened by him, which I don't think she really does until Crush. And then, freaked-out as she is, she doesn't want to have anything to do with him (or give him the pleasure of being the one who kills him).