Board Thread:Questions and Answers/@comment-4623180-20120919141837/@comment-5364525-20120920025244

I think he should have. The main argument was to keep it from Uther, but at that point, neither Merlin nor Gaius had any reason to believe that Morgana would say anything. Kilgharrah insisted that it was best that she not know even about her own magic because she was destined to join Mordred in an alliance of evil.

Yet, what does not telling her do but push her into an alliance with Morgause?

I think that Merlin should have listened to his gut in that instance!