The Belated Concern Is Worthless. Why Are You Crying When I Become the Demon Emperor?

[No system] + [Single female protagonist] + [Decisive killing] He was originally the proud son of heaven, the future master of the North, and the future successor of the Sword God. But he was thrown into the death cave at the age of eight. He grew up eating corpses to survive. I picked up an ancient magic weapon, the Ten Thousand Soul Banner. But for the sake of his family's reputation, he vowed not to become a demon. After carrying it for ten years, I finally got out of trouble and got to see my family I missed day and night. But... What faced him was not family affection. The mother doesn't care for him, the father doesn't love him, and he has a good brother left, so he kills him quickly. That being the case... Then why don't I become possessed? "Come, come, come, the whole family is reunited in the Ten Thousand Souls Banner!"