Of all the knights, Arthur expects Sir Leon to be the one that agrees with him. The man has been a loyal friend and mentor for as long as Arthur can remember.
It is, perhaps, why he spills wine down his front when the man refuses, even hypothetically, to arrest Merlin. Leon sets down his own wine. "At first I might have considered it," he says, "but Merlin has saved your life more times than I can count. For his loyalty to you, he has my loyalty and my trust."
Arthur doesn't sleep that night. He has a lot to think on.