I agree with the Prince's ideas stated motivations, but in the end he doesn't care more about the people than he does his own power. The steps he takes here could be more "justified" if he had tried talking to Anis first and she still refused to support him in changing the country. After all, he could still be king and just have Anis be his supporter and force through reforms. "The monster" doesn't have to rule the country, he just needs a monster supporting him. His reasoning for not wanting to be a puppet are that so he could protect the people from the nobles, but he is still not willing to be a "puppet" to his sister even if it would be for the good of the people, which shows that it is because he personally wants power.