[ Even as she speaks he's questioning if he really wants to hear this. Sure, he asked and the need to know is damn near killing him but at the same time she's giving him a truth he's not sure he can face. It was easy dismissing the idea to go to the good side when all he heard was kumbayah's and the constant disapproval of his very person, but Dawn has gone beyond that and damn well opened up Pandora's box.
It's weird how the one Scoobie he didn't know existed seems to be the one that know him the best.
He keeps his eyes on her, silently listening despite the way every new piece of the puzzle that clicks into place makes him feel... something. Sick, perhaps. Dizzy. Coupled with their last conversation it all makes such painful amounts of sense that he's damn near reeling under the weight of it all. It sounds so much like what he'd do. Go against the grain, rebel against any forces that seek to control him — he turned the Annoying One to dust when he didn't feel like chanting anymore and he went against his nature to stop Angelus from bringing about the Apocalypse even though it cost him Dru and even though he's been so dead set on gorging himself on Buffy's blood he knows somewhere deep inside that when it really came down to it, he'd just as easily fight on her side as he would kill her.