Ghastla didn't have other people around who would have been perfectly willing to follow any orders he gave them without "enhancing" them with their own sociopathy? And couldn't have done the stuff that would require advanced magical ability himself? There's absolutely no evidence to support either of those inferences. He's the ruler of an empire. It stands to reason that there will be plenty of people perfectly willing to do whatever the emperor tells them. That's the whole point of a frakking empire.

Furthermore, I'm quite certain that Celes explicitly cites Kefka's actions as a large part of her reason for her betrayal of the Empire in at least one translation of the game I've come across, so keeping Kefka around not only accomplishes nothing for him, but actually directly harms his interests by causing one of his best generals to defect to the enemy. It's entirely possible that, had he thrown Kefka to the side like he would have had he been smarter, he would never have had to manipulate the party in the first place.