It's one thing to leave her, he dated (and eventually married) the person he knows is actively antagonizes her. She, thankfully, doesn't know that he's doing that to gain status among the ranks for her. Which we, the reader, know.
The Duke is still shit because he's being cruel behind the scenes and even verbally abuses the ex-FL for what's really his own gain (this last chapter shows what flipped that switch and why he claims he'll get our current FL back once he gets the position he wants).
The Duke is not innocent and she's traumatized from being played. Everyone is against her and 2-3 guys left her for the same person that was supposed to be her best friend.
Following up with an addition to what I said earlier: re-read chapters 22-23. Like the ex-FL, he's a little shit who wants what others have and used to antagonize the current emperor
i'm looking at the scene in isolation. he didn't do any of these things before she belittled him did he?
He did. Go to chapter 22
i can see she flirted with him. What did he do?
I think you're mistaken when you say it's the same situation. Yes, MC was cruel in mentioning her boyfriend's rank, but she did it because it was reality. Neither of them had the power to stop the gossip. She didn't act superior either, and even said that they were the same.
I also think that comparing his betrayal to the same thing as dumping an arrogant partner is weird. He didn't dump her because she was arrogant. She didn't treat him like shit either. She showed him twice how bad her mental state was after being betrayal so many times, so he decided to betrayal her again. What human being with even an ounce of empathy does this?
Of course I understand that it was a "necessary" part of his plan and that he was "doing it all for her" (at least one can conclude that that's what he told himself), but I really think it was just an excuse. I think he's a bad person who just needed an excuse for himself to reach out for the crown. Besides, why didn't he say anything to MC? Because he knew she wouldn't want it or did what she wanted no longer matter? He is the villain here not because he is a guy, and in a story with reversed genders he would be idolized, but because he never even tried to explain himself to MC or apologize for emotional blow he dealt her. If any FL in any novel decided that she would betray her "beloved" by hurting him over and over again for years, people would eat her alive.The only explanation could be that 1) she had no choice and could not explain herself (he had years to do it and no one stopped him), 2) they were not yet lovers when she started her plan but she intends to confess everything at the next favorable opportunity (MC was his girlfriend to whom he confessed his love and he still had plenty of time to at least give some sign of his plan.), 3) she would feel like shit all the time (which is not reflected in the Duke's behavior).
Also in the situation you described, FL leaves her partner and finds a better one, while the duke intends to return to our MC when he achieves his goal and he doesn't even realistically think about the fact that MC might not want to come back to him because he hurt her too much. I don't know if you've read "Remarried Empress", but there the antagonist had very similar assumptions about the Empress. I think Duke is closer to that antagonist in his behavior than in the situation you described.
I agree with you, all i'm saying is with a little more perspective the duke can be conceived to be a seriously hurt person whose actions are designed to either hurt fl back or win her back, she doesn't know. And neither do we, conclusively. I'm not saying his actions were right, but I can see the driving force behind his actions were fl's words. Not saying that we need to root for this guy, and all his actions after were justifiable, but he isn't a 2-d character anymore. We can be sympathetic towards his actions.
You be sympathetic yourself
Imagine a story where a FL is dating a guy, and supporting him, loving him etc in every way she can. She knows of an old grudge the bf has against another guy (ML), who is very popular and has a bunch of girls swooning over him, but she doesn't care. She loves her bf the most. But then the bf belittles her for her rank. The FL decides she's had enough of being sniped at and decides to end things with her bf due to this and gets with the ML instead. In this situation, the FL is cheered and lauded by the readers, coz why should she stay in a situation where she is treated so shit? She will instead get with the handsome ML, and be the envy of all the girls around, and girlboss everywhere. The ex-bf then gets angry and vows revenge for her "betrayal", completely forgetting his own mistreatment of her.
This is the exact situation in the latest chapter, just with the genders swapped. The FL is the cruel and ignorant ex-bf character here, and yet everyone hates the Grand duke guy. Just some food for thought.