It should feel good seeing that hurt expression on his face but it doesn't. It wasn't really worth it, not with those words. but he's long past taking them back and that smal flicker of regret instantly vanishes when Jack can't resist dragging Spike into this. "Don't act like you would have stayed with me if he wasn't here!"
The last word is a snarl, a real actual animal growl and he feels his face shift and warp into something inhuman. It's shocking, mostly because he's never done that like this, not really. That angry, demonic ball of predatory aggression is always there but it's easy to ignore when you've had a soul from the begining and don't really know what you're missing. This is the first time he's ever wanted to hurt someone and for an instant all he wants to do is sink into Jack's neck or tear his heart out of his chest, not for blood. It because he wants to.
There's a long, endless second where he stares at him, visibly fighting it back. He can almost hear those words he yelled at him all those years ago in Cardiff when Lisa lay bleeding alone on the floor of the Hub. One day I'll have the chance to save you, and I'll watch you die! He's pretty sure Jack can hear them, too.
Then the moment breaks and he shakes his head, face melting back to normal so this time he can turn to walk away. "You're right. Why would I need you?"