Spike was prepared to kill Wood out of anger, blood lust, and as a simple act of self preservation. The simple fact hat he didn't actually do it had, I believe, far more to do with Buffy than with Wood or what happened to his mother. He wanted to show her that the faith she had in him wasn't misplaced, and that the mistrust that Giles, Wood, and the others felt for him WAS. A monster, even a tame one, would have acted on its primal instincts and killed Wood in self defense with not a second thought or a single regret. A good man wouldn't. Spike was no longer the monster.