He is as annoying as Prince Julius from "May I Ask For One Final Thing"....
The Difference is the dark haired Duke ML is not unfaced, super arrogant, overconfident and childish sadistic as that Prince.
They both are bored lonely miserable superior assholes.
It's because he found them amusing? And it's not like he knows she's being abused at home. He doesn't know her current situation in detail. He's just aware she's crown princess candidate who isn't attracted to crown prince and able to use sword well, so he's fascinated by that fact. Why got so angry? It's only 10 chaps. They don't know each other well right now and it's okay.
It's the fact that her face and demeanor are always serious ,and nothing about her gives the aura of amusement. So to me it just seems like he's mocking her in a away. Even the very first time they met he didn't take her seriously, just amused as usual.
The only expectation was the time she tried to cancel the wedding, it was a silly reason I'll give him that.
Sure he doesn't know she is being abused, but the situation the FL in is not amusing at all. First in the garden when she talk to the Crown Prince, the CP didn't even spare her a glance when she talk to him and only giggling with his mistress, only when she talk about canceling her marriage that he looks at her, no matter how it's a political marriage but it's not one sided (since the CP also has something to gain that's why he agreed to the marriage, it's not that he doesn't have a choice), at least respect her as human beings. And the second time in her house, her father and her husband to be blatantly ignored her presence and opinion, no matter how uninterested the FL might be towards the CP, the situation itself is very humiliating for her, but instead of wondering how could they treated her that way, he only interested at her sarcastic remarks.
If we take into account Lesian feelings and circumstances, none of all those witty remarks of her actually funny, it was her desperation to survived, buy how dare the black hair always laughing like he is watching some stand up comedy show!