As soon as R's hold slips, Atticus is squirming out of it. Part of him wants to stay and tear this creature to shreds, but there is still a small, rational area of his brain that tells him he should run, that whatever this thing is is not something he wants to fight. (Possibly, this is holdover from his extensive knowledge of zombie-related games/literature from his human life telling him he has no way to take off this guy's head, really.)

He snarls and snaps at R just a moment before he is taking off at a run, thoughtlessly leaving his clothes behind.