User blog comment:Druidinhiding/Alternate Ending Theories and Thoughts/@comment-24.236.138.13-20121228003856/@comment-5674726-20121228004951

Arthur's Bane was revealed to be himself.

I'd say that his refusal to accept the Disir's proposal to restore the Old Religion meant that Kara viewed him as an enemy and sided with Morgana, leading to her execution on Arthur's orders, and Mordred turning against Arthur and joining Morgana.