Rhetorical: If one is "nothing" with power, did one even had power in the first place and "deserve it"?
The prince always had power through his position, however limited it was as he was not the "crown" prince. His problem is simply wanting for more power without knowing nor understanding how to wield it. Then he thought he found a breakthrough potential in getting more power through magic. The mistake here is him confusing strength for power - power is the ability to influence behavior and decisions. And human societies are built on organizing people through institutional power/social contracts.
As it is right now, the prince is basically becoming the dumb bully people learn to despise and hate because he has some strength through his princely position and some magic talent, not because he can control and influence the environment and people around him.
And like a dumb bully, he found out there is someone with greater strength than him (our MC, Leon) and goes through some weird mental process before attempting some evil, stupid power trip actions to feel better about himself rather than correcting his course. If the prince befriended Leon, he'd have far more power and support in whatever the hell he wants to do.
The power here is magic. The prince older brother is crown prince but he didn't have magic talent.
lets say his older brother is talentless and stupid, and he is talented, smart and good in leadership so he have more power than his brother despite being second sons as people would support and rally behind him to make he the next king.
The problem here is, his brother is already talented and he is not. so he had no power in the beginning. his brother use his talent for good thus making him a great king candidate.
but later he learn he had magic talent while his brother is not. in his mind, he having advantage from his brother in succession battle as he had something special too. Yet, unlike his brother that use his talent for good, he use it to make him look superior, basically misuse it. had he already strive to be a good advisor to next king and learn leadership skill like his brother, even without magic, he is still valuable asset to the kingdom.
Throne didn't give you power, people do. if his talented brother suddenly have magic, its only add to his values. but for that prick prince, if he lose his magic, he would be nothing as magic is the only thing he had advantages from his brother. having someone else, especially commoner better in magic than him he choose to do evil instead befriend him like you said. so if one day if he losing magic, he become zero again, nothing. but if he choose to befriend MC and helps other people, even he lose his magic, he is still important person to them. he basically have edge to recruit people under him using magic as medium/ way to connect but he choose to be prick.
King only as strong as his loyal subjects.