It is not actually true that Spike has never been so angry in his life. Spike has lived a pretty long time, and he's spent a whole lot of it angry. Still, by any objective measurement, he's really pretty furious. He is at this point fantasizing about killing the slayer and her bodyguard very, very slowly.
He's mad enough to talk about it, too, the next time the government bastards send someone in. It's a dainty little lady in uniform, who looks very sympathetically at his injuries and asks him about how he's feeling. He screams and curses at her for a bit, but eventually he's able to talk himself down to what merely very furious conversation.
"What is it you wish would happen?" asks the girl, after he doesn't know how long of this. If he were more in top of things he would probably realize that this is a slightly weird way to phrase the question, but he isn't, so he doesn't.
"What do I wish? I wish I'd run into Karen and her pretty boy bodyguard back in England, back when Dru was at her best. See how he likes being cut into ribbons, right after I torture his slayer to death in front of him."
"Done," says the girl, who he now realizes is a demon, and suddenly his situation is looking very different.