He's introduced as a guy in the Empire (he's not really a soldier, more of a diplomat or advisor, I think) but you don't really hear much about where he came from. You don't really hear much about why he chose the path he chose. If I'm not mistaken, at one point Gestahl says that Kefka and Celes were given life to serve him, which would lead me to believe they had similar upbringings, and maybe grew up as a sort of foster brother and sister.
And through most of the game (prior to WoR), Kefka is a loyal servant of Gestahl, doing as he is ordered by Gestahl. Leo ordered him to be humane with the people of Doma, and he disobeyed Leo, but the punishment he recieved was a ruse, so obviously, the Emporor didn't mind that Kefka did that to the people of Doma. And Kefka continues to do as he is told by the Emporor. I'm not sure if the events in Thamasa involving Leo and the Espers were the idea of the Emporor, or if Kefka just did that on his own. But either way, the Emporor didn't seem to mind after it all unfolded.
Then Kefka loses it, by the statues. You know, the "Run! Run! Or you'll be well done!" part? Kefka seriously injures Gestahl, the guy who was something of a father to him, and then proceeds to push him off of the Floating Continent.
I was thinking it might've been that his anger snapped him when Celes stabbed him, and Kefka said he hated her a little bit. Because he flips out right then.
I was also thinking that maybe he realized that because of where he was, he had an opportunity he'd never get again. He had to take this chance to take the statues power and kill the Emporor, and then he'd have a ridiculous amount of power.
But it doesn't seem like Kefka wanted power. His goal wasn't to become powerful and then rule the world. Gestahl said there'd be no one to worship them if Kefka messed with stuff, and Gestahl was mostly right. Most people on the planet died right then.
So maybe Kefka wanted power so he could destroy the world with his power, rather than ruling it. It just makes me wonder why he'd want to destroy the world. I mean, Zemus was impatient, and Sephiroth was tyring to accomplish something "good", but Kefka really had no motive other than insanity. And if it was just that he's slap crazy, why did he follow orders and serve the Emporor for his whole life? Maybe he was always crazy, but was aware enough to know that the Emporor was far more powerful than he was, so he had to wait for the right moment to make his move.
I don't know, what do you think?