The look and implied censure have no effect on Derek. He continues to glare, with a quick break to visibly scoff at the idea that he'd consider a mountain lion reason enough to be having this ridiculous conversation.

"No."

The you dumbass is implied. Because, really. All this delay is doing is putting Scott in more danger, the very thing Stiles claims to be against.

(It would probably help if he said that aloud -- except the dumbass part.)

Eventually he remembers to add, grumpily, "I didn't bite Scott."