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She does make him care for her a lot but he was still manipulative and became obsessive, he was also violent. He killed Cayena’s nanny in order to keep her within his grasp, she gets really depressed and terrified. She wasn’t really able to change his personality. She decided to teach him a lesson and take revenge by becoming emperor and exiling him.

and this is far down the story but spoilers!
zeref is no ordinary bastard child, he's literally an unwanted child on both sides since he's the empress' and the duke's child. so he can't inherit the throne since he's not the emperor's child, but he also can't inherit the duchy either because the ducal couple divorced after some time and the duchy belonged to the mother's family so he's not eligible to inherit it. that's why i feel sad that if he could have just stayed sane, he could have lived his life as an emperor with his sister's complete support.
slight spoiler but on one hand, it's quite sad that he never changed even after being treated well by his sister but oh well with a history like that i can't blame him