From birth, Arthur Pendragon had been taught many things, but most importantly, he was taught to rule with his head and not his heart. Logic was more important than emotions; emotions tended to get in the way.
So why does he does his instincts want to pretend he didn't see his servant's glowing eyes? The golden glow told him more than what he wanted to know; it was the indication of magic. Vile, polluting, magic.
Merlin has practiced magic. But for how long? Maybe there's a way to save him from the grasp magic has on him. Arthur can't stand thinking about losing anyone else, and especially him.
"How long?" Arthur asks sternly, and his voice is sharper than it's ever been before in a considerable amount of time.
"Always," Merlin responds, the gold fading away to reveal his sorrowful blue irises. "It was never a choice for me, you have to understand."
But it's not even all about the magic. It's about the fact that Arthur thought he knew him, thought he could trust him. How much has been a lie? What were his true intentions, if he's always been in the heart of Camelot with magic embedded in his corrupt soul?
"I thought I knew you, Merlin," Arthur tells him, pained. He squeezes his eyes shut, and his heart clenches. "Who even are you?"
Merlin steps forward cautiously, looking at him with a fear in his eyes that shouldn't even be there. He's got power ready to be released with just a simple flick of his wrist, and yet, he has the gall to look scared. "I am who I've always been. I'm Merlin, your servant, your friend…"
Arthur flinches, the word hitting him with more force than he had anticipated. Everything is too much to handle, and he starts to listen to his head, deflects all the questions he still has. He stops listening to his heart.
His father's teachings come back to him. Magic is evil, and just by mere association, that makes Merlin evil. A thought that had previously been laughable. Merlin, a bumbling buffoon, his clumsy servant, but apparently also an evil sorcerer.
Everything Arthur does is for the protection of his kingdom, he justifies.
Head over heart, logic over emotions, and duties over friendships. A bond is broken, a man is executed.