The only explanation of Kefka's backstory beyond "he's one of the Emperor's generals" is offered up by a townsperson in Vector. If you're in the World of Ruin, you've already missed it.

Basically it amounts to the fact that Kefka was a magicite-infused soldier like Celes, except he was the first person to ever receive the treatment. And since it was still an imperfect, experimental operation at that point, he came out of it less than sane.

But really, Kefka's greatness doesn't come from his motivation so much as it comes from his actions.