Lacking knowledge of how exactly his mirror self normally handled Sora's behavior, Roxas couldn't be sure if he'd done anything that tipped Sora off about him not being evil here. It hadn't really occurred to him that his mirror self might reciprocate the name calling. Even if it had occurred to him, he wasn't so sure he could be quite that mean.

Or maybe he could. His frustration with the situation certainly made being civil difficult. Maybe that would help, "It might reduce the urges to smash that fat head of yours against the wall. And really? Bullying the pretty boy and bitching about what everyone else should be doing? Real productive, that."

At least sarcasm was easy for Roxas even when he wasn't in a bad mood. If he was lucky, that might help him here.