Kefka Sucks. Kuja Is Better

[Forsaken Lover]

You're kinda mixing up motivation with goals. The goal is power but power is seldom its own reward. It's what you hope to use that power for.

Kuja hoped to use the power he attained to free himself. Much of FFIX's storyline centers around the purpose of existence; why we are here and that sort of jazz. Kuja's literal purpose for being was to serve Garland and resurrect Terra.

Thus he attempted to gain power to subvert this. He hoped that, with his creator dead and the universe under his command, it would help him rise above what he was born. You can't be a mere tool if you're the strongest thing alive and everything else serves you.

That's why he flipped out at the end. Despite all his newfound power he was still just the same flawed creation. He'd never be able to establish his own identity and become who he desperately wanted to be.