LogFAQs > #976826457

Kuja and Seymour have some superficial vibes (mostly the crazy hair and piano themes) but beyond that they feel different in the context of the story. Seymour is introduced as an important member of Spira's society. He's even a party member for one fight. His heel turn, and the way his heel turn gets covered up and he gets protected, is used to showcase how corrupt Spira is.

Conversely, Kuja is an antagonistic outsider from the start. His first scene is basically him monologuing about how great and evil. And while he's technically working within society because he's 'allied' with Queen Brahne, you know that Brahne is bad news pretty early in the game. Kuja is a failure of Garland specifically, while Seymour is a failure of Spira as a whole.

That's just how they're used in the story. As far as personality goes - Kuja, even at his worst, still comes across as more sane than Seymour. Self-absorbed and destructive? Absolutely. But he doesn't give off serial killer vibes like Seymour does.