Like take this with a grain of salt. And if there are actual spoilers out there pls tell me what happens
Spoiler: the FL’s mom’s death…..(don’t read if you don’t want to be spoiled)
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So actually it was the ML’s mom who killed her mom. She was a very wicked woman and I don’t remember the reason why she killed her, probably some dumb reason. So when the FL found out she felt betrayed(since the FL and ML are childhood friends) because to her it felt that ML just let her mom die by the hands of his family when he swore to help her reunite with her mom. That’s why she hated him in her past life, she was very much traumatized and suffer from a lot of abuse from the count and countess.
I doubt the ML’s father killed her mother. I think that her adoptive parent’s (the count and countess) killed her mom, and then they blamed it on the ML’s father. That way she can direct her anger at the ML, weakening their relationship, and her adoptive parents will be able to control her more easily.