It's weird that you don't think so, they almost have the same reason.

Kuja (SPOILER)wants to destroy everything, because he knows he's going to die, and he can't stand that the world is going to exist without him, so he decides, if he dies, everything dies.

Sephiroth (SPOILER)wants to destroy everything because he knows he's already died. He can't stand knowing that he's just a monster and laboratory experiment, and in a sociopathic attempt to be reborn decides to 3/4 of the way destroy the planet so he will be absorbed into it as pure energy and emerge a god among it.

They were (SPOILER)Both created, both couldn't stand a revelation about their lives, and they took basically the same actions.

It's fine to like Kuja more or even hate Sephiroth. I just don't see your logic when you say he was uninspired and Kuja was.