@amazed_bun If I remember correctly, his mother's letter DIDN'T inspire him to be a tyrant. It was the opposite. I think she told him to not stand out and live in the shadows or something (I'll have to go back to find her exact wording), which is what he did in the previous life. He never became the emperor (nor tyrant). But he was the one who ruled the empire in the background.
EDIT:
Chapter 10: "Dylan rules the empire in the shadows". When people found out the truth, they blamed his mother who wrote ".. don't stand out, and you have to survive no matter what" in her will. He lived by his mother's words and didn't try to fight for the throne but decided to pull the strings behind the scenes.
Also having reread this I just realized that the herb wouldn't cure her but would only lessen her pain (so she'll probably end up dying anyway), so as
@Reina001 said, Charlize is probably trying to gain Dylan's favour.