User blog comment:Merlin2Emrys/Mordred - good or evil??/@comment-5674726-20121215101457

In With All My Heart, when Morgana told Mordred that Arthur would never tolerate him if he knew that he was a sorcerer, he told her that one day, they would be accepted.

I'd say that he will be loyal to Arthur as long as he believes that there's hope that he will come to accept magic. If Arthur gives him reason to believe that this hope is a vain one, Mordred isn't going to stay loyal to somebody who will never accept him for who he is. I wouldn't blame him for that.