He saw gold in his eyes, saw his attacker fall down dead and in that instance his heart broke. It was not because he was a sorcerer, but because he did not trust his Prince enough to tell him. They were master and servant, they were friends, they were lovers yet Merlin did not have belief in him. Merlin lied to him constantly. In that second Arthur knew that Merlin could not truly love him, not in the way that Arthur loved him. If he had, maybe he might have been more truthful. Maybe Arthur's heart would not have broken.