Let's correct it but i don't blame the marquis for not knowing tho.

She didn't commit suicide because she didn't liked herself, the situation didn't even allowed her to think like that. She did it because it was the only thing in her power that she could do to affect that man negatively, it was a form of revenge.

Also, I don't think she could ever remain sane if she didn't do it. Imagine having to keep spending the night with a man that killed your father, ruined your country, didn't protect your reputation or emotional stability at all and let you be 'bullied' by everyone, a man that is trying to tie you over with forcing you to have his child and on top of that, a man that doesn't even feel guilty about what he did and basically thinks that he treated you well by doing so, when he is the reason of all your traumas and mental damage? No one would be able to remain sane.

There's no way a duke with that high quality of a information circle (that maybe is even better than the royal family's one) would not know what she had to pass through. He may not know how much it affected her, they didn't even communicate for him to know and he can't read minds but he did know it affected her in one way or another. There wasn't a finger movement of his to try to help her, not even the thought of doing so.

It's not about liking or disliking yourself, it's about trying to 'save' yourself. Death was the only way she could escape him.