Imo, it's because she was essentially forced into this marriage. There's a few points that caused her resentment: 1. she had always wanted to marry for love, 2. ml technically deceived her and with holding information, 3. her forced marriage to a royal from another country means that she is basically ripped away from her family
#1 Marrying for love
Both the fl and ml have feelings for each other and we can see that clearly. However, she wanted to have a natural relationship progression with him. Properly falling in love then marrying. What happened had skipped all of that process and jumping into marriage. From what I can see, she feels betrayed that he knew what she had wanted and yet still forced her hand into getting married. It may not be his fault that she was forced into it but she doesn't know that and it's still true that she being forced into it.
If she doesn't marry him then her brother will go to war, putting his life in danger. So this leaves with two options, sacrifice everything she has (the comfort of her home, being apart from her family and destroying her ideal of a loving marriage) or be stubborn and possibly have her brother in danger when he declares and leads the war. Her hands are tied and we all know which is the lesser evil between the two options.
#2 Deception and withholding information
So ml technically lied about his identity. But I don't think this is significant enough. What actually matters is that he withheld information from her. Before she signed the marriage documents, she tried and gave chances to the ml to communicate with her. She wanted to know why is his brother suddenly proposing to her like that without his brother's presence? Why is he always locked up and not answering her letters? Why is his brother so adamant that she signs the papers first then she can talk to him?
In her situation, she doesn't know what's going on and nobody is explaining why. She tried to give the ml chances to explain himself but he blew those chances away (we know his brother burned the letters and blocked all communications but she doesn't know!!). She wanted to know where is ml, what were his opinions and just simply have a conversation but she was denied of all of that. She's frustrated and angry.
#3 Her world is destroyed
Fl is sick and has always been sick. She had never left the palace (I think that's what's stated? or she rarely leaves? gotta check on that again) so her entire world is technically just the palace and her family, including the servants there too. In her eyes, the marriage is essentially taking her away FORCIBLY from her family. It's destroying her world because her brother is willing to sacrifice everything to prevent her from getting married.
Everything is spiralling out of control and she doesn't really know what to do. Her only option is to accept the marriage to prevent her brother from starting a war and possibly getting injured at best and sacrificed at worst.
In this case scenario, she doesn't have control at all over her surrounding and her marriage. Her brother and her father are at odds with each other because of her, her soon to be brother in law is forcing her to marry the ml, her brother is willing to go to war. Everything is falling apart with her in the center of it all so I honestly think she needed an outlet to express her anger and the ml is the best candidate for that because aside from her, he's also one of the instigators that caused her world to fall apart like that in her perspective.
But this is what I think from what I've read so there might be more details that can be added or that I've missed! Hopefully more things will be revealed soon because the story seems promising
I understand that too but go into ml shoes too doesn't he obliges to listen emperor and get stuck in all this situation he wasn't in fault too but I really believe everything can be amended she was married to him for country right 1st instance but she has lot of chance to be happy in that marriage too please look at she has known about her illness she knows she won't last for more years of life she could have choose path be mend and choose have not happy but atleast be co companion to partner she get married to one time negligence is not stupidity but when she choose to ignore the surrounding that's where she went wrong i have read lot of other novels to i never find fl of her who dwell into illness and suffering and sadness . I do agree ml has his flaws too i admit but at end what he did was make him hero and make him male of this novel what she does that make her to be female lead it's more like now you will get chance to do things right when u choose to ignore make it worse in 1st hand it's not only her it's they both made their relationship worsen but point me one thing after their ml done wrong he always find way to make her happy and he respected demand not see him at all what had remained to be done i have never see people ding for others just because they liked each other but he loved her that's make him ml who stands for his action and take responsibility and face the consequences by losing his life in the end. P.S- i don't hate fl lead i just felt hurt and bad for ml , he was the only one who bears consequences of others and his doing and fl lead left with only regrets of negligence at 1st stance but choosing ignorance for the yrs of their marriage . Miscommunication and no communication leads them to doom.
My bad meant to dislike
lmao i was just confused though i wasnt hostile towards her or anything???? people just taking it the wrong way
im rlly confused in chapter 7 why did she even get so mad? like its implied she liked him too and she knew who he was and like didnt she know she would have to move away??? hello????