You only really get a sense of it in his EX trial.
His motivations are fine, it's just conveyed really poorly.
Which is a shame because if it
had been conveyed properly, he could've been a very compelling villain that a lot of people could relate to.
The simplest way to boil his entire character down would be to say he was just a spoiled brat with daddy issues, but you get a bigger glimpse into what's driving him to act like such a psycho during his trial along with some tiny, hard to spot crumbs along the way.
He was a miracle child prince who was born the son of a beloved King who united an entire continent under a single banner by traveling around, solving everyone's issues and finding a way for everyone to coexist. So from the get-go, there were high hopes for him and heavy expectations heaped onto him. Everyone expected him to be this amazing ruler who could even surpass his father. Having to live up to those expectations probably put a lot of pressure onto him to be what everyone thought he should be, instead of being who he wanted to be. If you're growing up and having people constantly tell you that you should be this and you should be that without giving any regard to your own autonomy, that can mess someone up mentally. Things started to get even more complicated when his dad decides to adopt 2 more kids into the family and suddenly, the perfect prince without equal now has 2 other "siblings" to compete with. 2 siblings that aren't even related to him by blood. Years of repressed jealousy and festering feelings of entitlement fostered by a lifetime of people telling you that you're expected to have everything builds up inside. Galool Ja Ja wasn't stupid. He could see what was going on with his son. But he was a fair ruler and wanted to give not only his own children a way to work themselves into greatness, but even others outside of his family. Hard work and understanding has more value than just using royal blood to lay claim to something that should honestly not just be given to anyone. I think Galool Ja Ja wanted Zoraal Ja to take lessons from the whole competition. He probably knew that he wouldn't win, but hoped that he'd at least grow somehow from the experience itself.
Unfortunately, it had the exact opposite effect. Zooral Ja was able to prove himself flawlessly in combat but when it came to more diplomatic stuff or things that required a softer hand, he failed miserably and it was a wake up call that he did not want to accept. Then it culminates into what he perceives is his biggest humiliation: getting his ass handed to him by a
shade of his father. Not the real Galool Ja Ja. A
phantom. Zoraal Ja could never defeat his father even as old, half dead. and weakened as he is. So when he fights a conjured imitation of his father in his prime, he's absolutely destroyed and mocked for his weakness. Suddenly he can't fall back on just martial strength anymore and his pride refuses to allow him to set aside his personal hangups to work with others. It's the straw that breaks the camel's back. Feeling like he was purposely set up to fail, Zoraal Ja does away with any form of honor, and just resorts to underhanded tactics. This isn't just about proving himself anymore. Now he's just doing whatever he thinks is necessary to claim what he feels like should naturally belong to him, justifying his actions with the thought process of everything belonging to him by right and that he, as destined for greatness as he is, shouldn't lose or acquiesce to those he believes are inferior to himself.
In comes AI Sphene who basically gives Zooral Ja everything he needs to be able to act out his insane intrusive thoughts and he happily steps over the line into a place now where there's no saving or helping him. He can't be talked out of what he's doing, he can't be reasoned with or dealt with the way a lot of villains before him had been dealt with because stopping now would mean everything he's endured and done would've been for nothing. Talk no jutsu is off the table. Now, given free reign to behave how he wants without having to hold back, he goes completely off the deep end and it only gets worse
after he murders his father because while he does manage to kill him, he could only do so by relying on extra help from bascially cheating death to cheese the fight. That doesn't make him feel confident or better, that just angers him more. Yeah, he cut him down in the end, but had he not had fancy gadgets to help him and literally bring him back to life, he'd be dead on the floor and DT would've ended right there. Now he thinks there's no other way to prove that he's a true ruler than to basically subjugate everyone and everything in some desperate attempt to prove to himself that he isn't just all hot air and to prop himself up to be a better ruler than his dad ever was while he childishly lashes out at anyone that would question those claims, completely missing what it actually means to be a good ruler.
Yes, Zoraal Ja was just a spoiled, petulant child who's response to critisism was to just murder you, but there is some nuance there if you look hard enough. Not saying that he was a great villain or anything. It's just, he could've been better and was greatly hampered by the poor writing, much of how all of DT was.