This is just from my wonky memory, so sorry if I am wrong. in the story that I read, the emperor was an okay person I guess. But the sister has the ambition to be emperor. She can't because she is a girl. So she wants to control the little brother. The way to control him is through poison. So her poison is making him easily angered and whacky mood so he became a tyrant. But this tyrant feels calm and good physically because of the poison perfume the sister wear. Thus he listens to her well cause she is his only source of comfort. So think of the poison like drugs, you feel good when you use it but after that, it will destroy your body