Honestly, the young king had no reason to be scared of him or his magic. He did feel a little bit betrayed by him, but knew that with time, he'd get over it. Watching Merlin's expression change from the worry that he had in his eyes to confusion, Arthur just wanted him to say something - anything - to show that he hadn't ruined their friendship.
He started to think just that when Merlin spoke, and all he could do was nod slightly. "Yes, I was, and still am in love with you. Everyone knew, even Guinevere," he said softly, right before Merlin's fingers found their way around his cheek. As he felt their lips meet, Arthur's heart soared, and he couldn't help but snog him right back. Pulling back after a few moments, he rested his forehead against the other's as a small smile appeared on his face.