
basically, he was an orphan as a child, and he was forcibly picked up as child soldiers and trained ruthlessly. when he grew up and fought with his colleagues for a mission, his colleagues died and fl's father knew that the mission was deadly, so basically fl's father purposefully doomed his colleagues (aka the only bonds that ml ever made and cherished to a certain extent). ml survived and he met fl, and she talked about the soldiers' death as if it were needed for the country to thrive, even though their deaths were driven by fl's own father. taken from the webnovel translation:
"
“Yeah...just me, I'm the only one who made it back alive. All of my friends were in the operation …”
Like an idiot, he couldn’t speak well. He couldn't c ount the number of times he had acted as a spy, and yet he could never quite control his own emotions at this moment.
"Yes. The people who gave their lives for the country. My heart goes out to them.”
Annette looked not at all mournful, and said so as if their sacrifice was natural.
“By the way, when will your ceremony be?"
For an instant, time stopped.
"
So basically the way fl said it was hurtful because his colleagues died and she simply brushed it off as sacrifice for the greater good. imagine being heiner and having basically no one that he can rely on and has a good bond with except for his colleagues, and they died so ruthlessly. and the way fl said it was so ignorant that it was hurtful. yes fl doesn't know anything, but sometimes ignorance hurts the most and for heiner, this was really conflicting with his thoughts about her because he already loved her at this point but he was so hurt by what she said that he became twisted.

Isn't this due to misinterpretation? Because FL did not know she was saying it like a cold manner, and she meant it out of good will. I haven't exactly read the novel, so I prolly don't know lol
But personally for me, even if she did say that and it was offensive to him, did he not speak up on the matter? Yes, he's allowed to be angry, but not at the expense of hindering his own and another person's life by revolving around that one statement that may not have been meant to be harmful.
I feel like if he had communicated that with the FL to that time she had said it, then perhaps things would have changed drastically. But once again, I have no clue since I haven't read the novel LOL

he can't be angry because fl's dad controls everything in his life. fl's dad could literally kill him anytime because he's forced to be a soldier by her dad. he had to be nice to her, and communicating that would reveal that her dad is evil (and at that point fl wasn't close to him yet so she probably wouldnt trust him yet). and honestly i don't think it's misrepresentation. although i think fl genuinely was just being apologetic about the soldiers' death, but the way she just said it was a "sacrifice" and then immediately changing the subject to ml's promotion makes it seem like she's brushing off the matter (ml was promoted as general because he survived the deadly mission that killed his colleagues). i think even if she were to said it out of good intentions, the way she said it was insensitive either way. someone died and she just pushed it out of the conversation, and i think that was definitely her mistake. and that mistake affected him very much, and paired with his hatred towards nobles, he began to have a deep-seated rage towards her. BUT at the end of the day i think ml is just twisted and psychotic and i think his actions towards fl were not valid and extremely harmful.
i've read the webnovel and honestly i don't like him better lol. also some spoilers...
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.i felt bad for him because he was deeply hurt by what fl said, though fl said it out of ignorance. but fl's ignorance is the reason for why he ended up bearing a deep grudge. what she said was so hurtful without her knowing, and i understood why he ended up so twisted (though his own abuse against fl is inexcusable and i see his actions more of a projection of his anger towards the wrong person, and in this case fl became the ultimate victim). The webnovel i read is definitely not a romance webnovel, it's more of a story of war and how annie learns from her ignorance and how she learns to heal. it's also the ml's own reconciliation with his tormented past, which is deeply deeply disturbing, and fl's father played such a big role in making his life literal hell. not even kidding, his life was hell. but yea i did not want him to end up with her but they did end up together!!! So for anyone who doesn't want that skip this immediately lol