Since we know that the set-up was a misdirect, the line "bitch is gonna see a change" is pretty obviously part of the writers' "plot" to "confuse" us by making us think Spike wants to hurt Buffy. (Which again, remember that he already *can* with the chip intact).
So, when he says "bitch" here, aside from basically bad writing, I think it's more to do with the projected anger that he feels toward what he did. He's disgusted with himself; that's clearly shown all over his face in the aftermath of the AR. But because he's a vampire and utterly confused by the guilt he ('s not supposed) to feel, he projects the rage onto Buffy, essentially blaming her, and we have that "bitch is gonna see a change" sentiment. Credit to [MENTION=12223]EffyStonem[/MENTION] for thinking of this, btw--it hadn't occurred to me, before.
Other instances of Spike using the word "bitch" I wouldn't call cute, but in season 5, "Out for a walk...bitch," certainly is funny. She said "five words or less," and he answered in 4, adding "bitch" because she'd given him such a difficult time about it. Believe me I'd never be ok with a man calling me a bitch to my face but somehow a soulless vampire doing it is sort of akin to a human being sneezing.
And of course "I'm free if that bitch dies...I'd better help her out" is such a telling line. It's truly a foreshadowing of the events to come in Seeing Red, I think. As I was saying before, this soulless vampire feels so much affection for a human being and on some level, genuinely regrets it and resents her for it. A lot of the time, we only see how devoted he is to her, or toward impressing her. This is a reminder that, because he is in fact soulless, instead of delighting in his love--basking in it--he is terrified by it and by the fact that it keeps him in line, doing the things Buffy wants him to do, saving Buffy instead of doing what his nature tells him to do: hurt her, or at the very least, make her life miserable.
In a way, "I'm free if that bitch dies...I'd better help her out" is very much what is going on in his head when he goes to Africa. After the AR, he realizes just how much he cares about the possibility of losing her, and just how frightening the concept is to him that it could ever be by his own hands. It would be so much easier to not care; to "be a monster." To let her die, to physically torment her like he should want to do. But he's certain now that he "can't be (that) monster." But can he be a man? Not without a soul; he doesn't quite know why, but Buffy has shown him that that's the case. So the best option is to get that soul and to "help her out," which, while soulless, he thinks essentially means becoming like Angel--becoming her partner, in a romantic relationship. After receiving the soul, he realizes that it means being there for her, without the right to have other expectations, in light of what he'd done to her.
