That is something that the comic book writers have now decided is cannon, (and grrrrr, argh, Buffy is a murderer, etc), but as of The Gift, the MO of Buffy and co as it related to the morality of slaying was vampires yes, (unless chipped or souled), humans no.
Personally, I don't like the idea that Spike is special. It's baseless and it diminishes the character, hell, it completely ruins him.
There are two main ways in which Spike is different: One, he has a chip; Two, he fell in love with Buffy. Having the chip wouldn't have stopped him from being evil (as we see in As You Were), or even from getting yet another doctor from getting the chip out. Obviously the reasons for his lack of action were the writers' need to keep him on the show, but we don't like meta explanations, do we? So we put it down to (a) stupidity, and (b) his personality--he's different. His lack of action demonstrates he's 'special'. Angelus, for example, would've just talked people to death First-style, or just gone for mental hurt and manipulation the whole way through.
Spike helps Buffy not because he's good but because he has a good enough reason to do so
Yeah, no one is contesting this. It is absolutely true.
He realizes that his chip doesn't work against Buffy, what's the next thing he does? He attacks an innocent. How's that for a change? No, Spike never changes.
He's still evil. He can never be truly good or selfless without a soul. There's no way for that kind of change to happen. But while he doesn't change completely, he does change. Actually, there are two ways of looking at it: that Spike himself doesn't change, only his motives do (i.e. the women he changes for); or that Spike does undergo a dramatic change--he is unable to be a man or vamp, and manages to changes (or chooses to gain the ability/the choice to genuinely change) in choosing man. I can't argue against either because I think both are valid.
