Kefka was just a man with a lust for power. Sephiroth may have succeeded in setting fire to a village and summoning meteor, but Kefka killed a whole kingdom and the only adults left by the end of the game were the main cast themselves a long with Katrina who was pregnant. Basically 94% of the worlds population was gone, and the planet was nothing more than a huge ocean and the lands nothing but desert. Though human, he was a monster. He would kill any one and every one and not give two sh*ts and a rats ass of who and what you were. He succeeded in becoming a god, but of course, like all bad guys, he was defeated.
He also had his own cult.