In Season Four theres an amusing subversion of the "old villain returns and has a sinister monologue watching the heroes" trope where he starts and immediately gets stunned and captured by some military guys called the Initiative. They put a chip in his head that shocks him when he tries to do violence to humans. He escapes and appeals to the Scooby Gang for help, they think helping a sociopathic vampire be able to kill again is a bad idea, so he mostly just hangs around for the rest of the show being their frienemy (he finds a loophole that he can fight monsters and demons without triggering the chip, and in later seasons goes through a serious character arc of his own).

It was mostly a lazy plot device to get a logical excuse to keep Spike as a regular,, he fills a niche of being a ruthless soulless vampire (in contrast to Angel) who could both play Devil's Advocate or be comic relief depending on the situation.