I know that Arthur didn’t actually know about Merlin’s magic, because his reaction to finding out in the final episodes makes it clear that it caught him off guard.
But then there are scenes like the “there can be no place for magic in camelot” scene, where you just have to think… he knows. At least on some level, maybe even subconsciously, Arthur knows. Because he looks at Merlin, looks him dead in the eye while Merlin is saying that magic is evil, and he says “maybe my father was wrong, maybe the old ways aren’t as evil as we thought.” And Merlin is crying, and he’s trying to hide it but it’s still obvious and Arthur is just watching him, waiting for an answer.
And then, in what is a defining moment for Merlin and Arthur’s relationship, whether you view it as romantic or platonic or what, he asks “what should we do?” Not “what should I do?”
What should wedo?
He’s putting himself on equal footing with Merlin; or, really, putting Merlin on equal footing with himself. He’s no longer simply asking for Merlin’s advice. He is trusting Merlin to make a decision.
And you can tell what he thinks Merlin will say. “Magic will reign in Camelot once more. Is that what you want?” Because throughout the show, Merlin has been a staunch defender of magic. He has openly sympathized with druids, played a hand in saving sorcerers from the pyre, and argued with Arthur about how not all magic could possibly be evil. So Arthur, of course, thinks that Merlin will tell him to legalize magic, save Mordred.
And instead, Arthur watches Merlin cry and say “there can be no place for magic in Camelot.” And the way that Arthur looks at him, it’s so clear, that he knows.
