She has never been a good woman. Virtue is not in her nature. She is selfish and avaricious and ambitious; she is led, always, by her own desires and her own self-interest. Currently, those align neatly with Richelieu's; one day they will not, and she has plans for when that day comes. (She does not under-estimate him.)
She is not a good woman.
But she is also not an unfeeling woman, and by God, she wishes she were. Life would be easier; tidier. Sometimes she wishes she could simply tear her heart from her breast and never feel a pang again, not of love or longing or sorrow or even hate. How beautiful it would be to be free of those things, how simple, how pure.
But here she is, not free, but chained endlessly by the past she is unable see beyond. She hates him so much. So much she sometimes cannot breathe for it, so much her hands clench into fists that she cannot control, so much she can feel the bile burn. He should be nothing to her, nothing, no-one, just a lover (a husband) from years ago, long-forgotten, his image crumbled to dust in her mind's eye. She should be able to walk past him and barely recognise him, to see his face in the crowd and feel nothing.
She pretends, of course. Pretends to Richelieu that she is indifferent to Athos. Pretends to the men she seduces that it is they for whom she burns. Pretends to herself – oh, many things. Athos means nothing to her. He has no bearing on her actions.
Which is why, of course, she finds herself back at the chateau where she was once the Contesse de la Fère. She burns the place to the ground and pretends to herself that it is merely self-interest and pragmatism and nothing to do with the visceral urge to destroy a memory so painful that she cannot bear to think of it. She burns the place to the ground and pretends to hope that Athos will burn with it (and knows that his young friend has come and he will not).
She rides away from the flames, into the forest, into the dark, and pretends that it is done; it is gone; he is forgotten.
She loved him. In the forest, in the dark, riding away, she cannot pretend. She had loved him, her perfect husband. In his eyes (in his arms), she had been reborn; a woman without a past, without sin. She had had a life of love and light. They had loved each other, and it had been perfect. Until Thomas. Until he had threatened and lusted and she had realised that she was the woman she had always been, prepared to do anything to preserve the things she wanted. And thus she had lost them.
She rides away from the flames. She can still feel the weight of Athos in her arms, her hand drifting to his hair in a reflexive gesture she barely noticed. She can still see his face, tormented and confused and she hates him so much, wants to hurt him so badly, because he didn't love her enough.
She thinks, in the forest, in the dark, that she had loved him too much and he had not loved her enough, and it had damned them both. But she does not care. It is in the past, it is nothing, he is nothing. She is ruled by her own desires and her own self-interest, and that is all.
(She still desires him.)