I think her hatred for him originally stemmed from the rumors about him. Remember in the earlier chapters the captain kept warning her to stay away from him because he was a cruel man. So even before she got to truly know him, she already had a bad impression of him. And then how he handled the brown haired noble girl just reinforced the rumors being true (in her mind). It’s literally just one big ass misunderstanding about him.
I think not only was she influenced by the rumors and the things she has seen while being his knight, she's more so mad since she was so anxious about the pregnancy but he knew all along and let her suffer alone, plus the whole dream manipulation. Although to be fair, she didn't want to tell him so idek
I really am asking this because I feel so confused at this point. Why does the fl hate the emperor so much? Besides the dream stalking(?) thing what did he do wrong? The fl says that he mistreated her but he was trying to treat he so well while she was pregnant. I just fell like I missed something ╥﹏╥