"Yes, the chip's working." He knows Jack will ask how he's sure, just like he knows he isn't going to like the answer. "He's in control of himself, Jack. He knows what he's missing and he wants it back. He isn't going to hurt anyone, even if he could."
Not unless they hurt him first, which is what he's most worried about. If this turns into a confrontation Spike is the one who has the most to lose from it, not just his self-control but also his desire to get his soul back on his own if he has to.