She's an innocent, kind person twisted by her parents to lose everything. She found true love with the crew, but her traumatic experiences made her borderline insane; one slip and your life's over. She doesn't have the compassion to not kill, that's not in her nature.
Don’t know if I have as much sympathy knowing she experimented and killed all of her boyfriends now then when I just knew she was abused by her parents
Nier deserves some slack. Her parents used her to acquire prestige and discarded her in the dungeon of their mansion with the muck and rats. Many nights Nier spent down there, listening to the glee of her parents and her sister Corrie celebrate a life *Nier* fought for. As the ultimate *fuck you* they sent her sister down to the basement to murder her, because they got sick of hearing her tortured screams and crying.