User blog comment:Morgana High Priestess/The alliance which destroyed Camelot!/@comment-5674726-20130108001844

I'd say that they handled Mordred's changes of allegiance quite well, all things considered. Morgana risked her neck to save him when he was a child and he bonded with her but, when he saw how unstable she had become, he decided to side with Arthur instead.

He trusted that, one day, things would be different and those with magic would be accepted and held on to that hope until Kara was executed, despite his plea to Arthur to spare her. It's important to note that Mordred didn't know that Arthur offered Kara a reprieve. He saw her taken down to see Arthur and the next thing he knew, she was being executed. He was angry and believed that he was wrong about Arthur ever accepting magic, so he left. Maybe he would have stayed out of the war between the Pendragon siblings under other circumstances but joined Morgana to avenge Kara.