"You're... I.... That's not even fair! I didn't even know!"

Arthur does look guilty, the fact is even now, he has trouble believing Morgana could have magic. But Leon, the knights and Guinevere had seen proof. To say nothing of the collasped tower still being repaired.

He shakes his head. "Father would never have killed you. He loves you too much."

But even he doesn't sound too sure.