how did the first prince become so evil??

That's my first question too.

What's the crown prince character development that make him evil?
His parents are good couple and his position to be king is secured.
Bro, what he really wants so bad that he burn our mc parents and abuse his siblings?

Sometimes people just have infinite greed. Also, their parents seem good but I find it very questionable how they don’t know that Franz is being abused even though they dote on him. Like, that’s not possible without being some sort of neglectful at the very least