Arthur is in the heat of battle and all gun-ho and then he turns and sees Merlin and he just turns into a little kid. He looks so devastated and just stares for a moment like he isn't even comprehending what he is seeing.
She was the darkness to his light. Her insanity was so perfectly paralleled to his sorrow. Merlin loses everything, Morgana gives it all up in pursuit of power. I love Merlin and Morgana they are my two favorite characters.