This has all caused Cade a lot of anger and upset, but somehow, he wasn't completely hurt by it until just now. He isn't even sure what did it, but perhaps simply the notion that Alistair... wanted him to hit him. He wanted to fight, but wouldn't unless it was Cade's fault. So Cade would take the fall. Cade, who had so few chances left, possibly none.
All this, from his first friend. The realization gnawed at him, draining all his resentment into cold defeat. Alistair wouldn't mind if he went away forever. Alistair would do it himself.

Cade looks at the floor, the apple in his hand forgotten for now. "...I don't know.. why," he says faintly. Why any of this. Maybe Alistair has known all along, and is taking the opportunity to express his disgust.