They seem to think that the pain will be enough to stop him. They forget, if they ever knew, that pain is something a body can become accustomed to—and they don’t know that for the first twenty years of his unlife, he belonged to Angelus. No human invention will ever equal to the pain Angelus taught him to enjoy.
Once he learns the triggers, the tiny piece of metal and electricity is easy to circumvent. He is not the creature he was before, but neither is he helpless. He hunts for fresh blood, but he leaves the humans alive, and the Slayer never knows.
She is simple to fool, only giving faith to what she sees. The Watcher should know better—a grandchilde of Angelus, felled by scientists? But it’s been so long since a true reign of Angelus, the Council has forgotten. Those months of madness culminating in Acathla were not Angelus, his mad, brilliant grandsire, only a pale shadow.
He lets them believe he lives by their sufferance, never giving a hint that the muzzle in his brain only offers a slight hesitation anymore. He is learning to navigate around it.
Four months after his escape, he kills a man on the street, and the chip doesn’t give so much as a jolt. He feasts that night, painting himself in blood.
He leaves Sunnydale after cleaning up the mess. He’ll be back, though, to teach the girl-child Slayer why a true hunter never leaves an enemy chained.