From what I remember from the novel,
The 1st prince has an inferiority complex toward their talented other siblings, and was originally kind to Al b/c it made him feel better that there was someone worse off than him (thanks to Al's magic affinity). Then Al made 2 spirit contracts at once, proving he was just as talented as their other siblings, so 1st prince's inferiority complex again reared its ugly head and he started being an asshole to Al. Additionally, 1st prince has always felt pressured by everyone expecting him to become the king, despite not wanting to and feeling like his siblings are all way more qualified.
Oh damn so he is just an insecure ass hole from the beginning (╯°Д °)╯╧╧
Over throw that Prince and become the hero!!!
Love the King which is rare, it even seems like the "cursed child" might be his favored one to be king maybe ? Its refreshing to see a Royal Family who act like a regular family. Altho Im curious how the 1st prince went from defending him to accusing him like that (╯°Д °)╯╧╧