He stumbled back a few steps, steadying himself before taking a step forward again. Not raising a hand against Arthur. Let him get his anger out that way, if he must. Merlin could handle a few shoves.
"How did I know you wouldn't change your mind when you found out I was a sorcerer? You didn't stop them building the pyre when Gwen was accused of witchcraft." Seeing Gwen carted off by the knights for magic he had done was one of the worst moments in his life. He hated the idea that she had suffered for an instant because of him, that his recklessness had nearly got her killed. Worse still when it seemed Arthur would let it happen.
"And if I did, and you kept my secret -- Every time your father talked of sorcery, or he sent you out on a hunt for people using magic you'd have to lie to him. Could you do that? Would you want to?" Merlin rambled, frantic to explain himself, and make it clear he really couldn't. He was thinking of Arthur as well as himself. He was thinking of Gaius, fearful that the physician would be put at risk for protecting him.