Oh, I agree that Buffy would never just walk into his crypt and stake him. But I certainly think that Spike has given her enough reason to do so without feeling like she's reneging on her deal*, by proving that he is a.) still capable of doing evil, b.) actively trying to get the chip out, and c.) would kill her as soon as he had the chance.

*That is to say, I would not feel like she's reneging, not necessarily that Buffy would feel that way.