"I knew the most dangerous woman in France," Athos informs her with an edge of implication that she cannot be worse than Milady, she cannot be worse than the woman he'd thought he'd known. "You were given power. She was given nothing and she let that darkness consume her, allowed it to drive her to horrible things, and doing it to the people she claimed to love."

She had murdered Thomas and for that, Athos can never forgive her.