Kefka was evil.
But so was Sephiroth. Someone who believed it his divine duty to wipe out the entire population of the world.
And Rufus. Someone who would do anything, for the ske of money. But his father was more so, and truly eveil. Bringing down the plate on his own citizens - that is evil. In fact, it is more than being insane like Kefka. He did it cool and calculated, purposfully and of his own free will.