User blog comment:Morganaforever/Episode Review: The Drawing of the Dark/@comment-5845325-20121215222012/@comment-5829707-20121216030422

Merlin told Arthur Mordred's plan to escape with Kara because he wanted Arthur to bring Mordred back and let Kara go. The idea was that if Mordred were allowed to run free, he would side with Kara and Morgana and it would be all over. Merlin thought that by keeping Mordred close, it would prevent him from turning against Arthur.