In all honesty his statement before the final battle I think sums his ideas up the best. "Why do you hold on to life. You can't live forever so why do you try."
This tells me that he feels that since we are all going to die he just as well speed the process along by killing us all himself. Truly a diabolical and great Villain. Really Sephy may have been "EVIL" in some peoples eyes but he is PATHETIC when compared to the heights at which kefka rose only to have 12 adventurers smite him down after over a years dominance of the planet. I must admit the final battle with kefka at least holds a little more challenge to it the first time than Sephy did too. Sure you can use the broken genji-offering combo and whatever but first time thru VII you just start throwing 12 KNIGHTS of the Round up at him and he falls truly a pathetic final encounter with your VILLAIN.