Agreed that Buffy keeps her word (at least, in the show - in the S8 comics, she kills demons she's made deals with without a second thought). On the other hand, Spike proves several times, notably in making his deal with Adam and in OOMM, that simply having a chip in his head does not make him harmless. And I wouldn't have faulted Buffy in the slightest if she had staked him, then, or at any time, really, up to "Intervention."