Originally Derik was prepared to keep plotting against the royal family and gaining the trust of the people while making them loathe the royal family but in just a moment he changed his mind on how to do things. In his past life, he was never the type to plot against an enemy, he could barely remember the last time he did.
When threatened back there he just confronted the enemy head-on and resolved matters with his fist. Choosing a different method of dealing with enemies gave him a strange feeling that made him uncomfortable. He thought of the future after he takes the throne, then what? He was never the type that loved ruling and even if he did, he would draw too much attention. Dealing with a kingdom as small as this shouldn't be an issue for someone at his level but dealing with the empire is something else entirely.