I too would have so much resentment for everyone. She was tormented by the adults around her, and instead of comforting and protecting her, her mother chose to cold to her instead. (Even if it was out of love, for a child, it must've been a cold rejection). Instead of having a loving relationship with her half-siblings and step father, she's ignored and bullied instead. Though I understand the half-siblings' resentment for her.
The only person she considered as "salvation" of some sort, her half-sister's fiancé, turns out to be her enemy.. I would build walls too.
And finally seeing her younger brother get treated well while everyone hates her, when they were born from the same mother is unfair to her. I can see why she would develop a sense of inferiority. She had no one to rely to but her own. And she's not even reincarnated or didn't went back in time.
I know she holds so much resentment for everyone, and I truly can't blame her for her actions. If I were in her shoes I would react that way too. My baby just wants to be loved. This manhwa makes me cry so much.
I was waiting for a villainess manhwa where the villainess is actually a villainess. My poor baby went through so much, and honestly I can see why she acts that way. She was treated badly from the start.