As the biggest Kefka fan to ever live.
I don't really consider him as evil as much as insane though. I mean if I went around and killed lots of people due to my insanity, people would say "that persons crazy" "that persons insane" not "that persons evil".
Then again, I believe that there are people who find that killing other humans as a method for entertainment and get the pure joy out of it. I don't consider them evil, more power to them for finding their joy in life.
And no, Kefka's purpose doesn't come into WORDS until the final battle but in hindsight, Kefka does have a purpose. Kefka believes that all life is meaningless thus he must exterminate it. Anything having to do with life, love, happiness, he wants crushed because he believes it has no meaning because it will all be destroyed in time. People die, People lose love, People will lose happiness at times. People give things meanings, and they don't know what to do when the meanings are lost, therefore, it has no point. If it has no point, he wants it to be dead.
The towns he put his light of judgment on were towns that disobey'd him. The second someone turned against them, their point was lost, they were not loyal so they died. Once Kefka was able to use Ghestal to get into power, he killed Ghestal, Ghestal no longer had a reason to live. Leo was meddlesome, annoying, and a bug to Kefka, thus serving no point because Kefka knew he'd have ultimate power, thus losing the point to live, leo dies. The planets gonna die anyway, so might as well take it over before everyone dies too, Kefka takes over planet.
This is another reason that Kefka in lore is "The angel of death" or the epitome of a backwards fall of satan (See final battle map), The angel of death comes to people or places when the people or places have basically exhausted their reasons or points of existance, then takes them to their respective places. Unlike the angel of death, Kefka choses when the people have exhuasted their reasons, then kills them in a violent fashion.
But then again, because Kefka is able to do all this in a semi-tactful way, it would show he had the mental capabiliites to plan and to know what he was doing, but that doesn't mean that he didn't find everything he was doing to be correct, because he did. Insane by our terms, clearly sane by his own. In fact, we're the crazy one's for thinking we have a reason to live, or that we will be happy in life.
I don't consider his reasons to be evil though, there are many people currently alive that have the veiw that everyone is pointless and should die, and everything will wither in time, the only difference is, some people act on it some don't. Thus we get the killers and such who believe they are perfectly sane and they were doing the killing, not because they were evil, because what the killed had no purpose.
Most people say that his motives changed in the last battle, that he just decided that he wanted everything to be destroyed because it was going to happen anyways, infact, everything he had done previously in the game was justified or finally given reason, in the final battle.