power didn't change you, it just show who you really are. he said prince used to be good, that because he didn't have a power. when he had magic, he wanted to show how superior he is to become crown prince instead instead being humble and appreciate the facts that he have magic.
if you are nothing with your power, then you don't deserve it.
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.