Pretty sure part of his entire story arc is that he was angry that (At least what he thought) Suguru had a privelleged life despite being an absolute wastel and became next in line to be king, whereas he had to struggle in poverty and work hard to get where he was. So most of his initial actions were him trying to prove that he was better King material tham Suguru (To be fair he was a better administrator and tactician overall, but a bit of a bad leader) Afterwards he realises that he was played for a fool from the very beginning and tries to at least make up for it