He had literally JUST told Norman that if he ever came at him again that he'd get handled. Did you miss that?
No, just different interpretation of the wording. "Coming at me" implies face to face with your fists. From what I inferred, anyway, when I watched the first time. Cold cocking someone's skull from behind with a mallet seemed to take it to a new level. Dylan's words didn't match the shock on his face. He looked particularly....surprised. His eyes were so big.