We should not really jump to conclusions about what Kefka was like before the experiments. We can't really assume that Kefka was already a narcissist before the experiments.

He definetly was a general, if you think about it, you fought Kefka countless times throughout the game. Sometimes they were comical but he seems to have the type of personality where he would toy with his prey.

He is a sociopath who basically views himself as "superior" to humanity. Now, it is true that Gestahl said something rather paternal concerning Kefka and Celes, so chances are we have a similiar scenario as the one in FFIV about Cecil and Kain regarding their relationship with the King of Baron. But I could be wrong and the two could have volunteered(sp?) but Celes was infused with magic as child, what about Kefka though?

I don't think Kefka hated or resented the Emperor. He never seemed discontent about Gestahl gaining the power of the Goddesses. If you remember, Kefka didn't attack and kill Gestahl until after Gestahl tried to kill Kefka for disobeying orders. Kefka felt betrayed and at the same time unlocked the power to become the god he always knew he was.

I do think it's interesting that Kefka is the only villian who had a rational explanation for destroying everything. X-Death was consumed by the power of the Void which is a spell that can only destroy, and Kuja wanted to end existence cause he could not bear the thought of the universe going on without him. Kefka on the other hand, after spending time as god for awhile just came to the conclusion that nothing really mattered. Life, no existence itself is meaningless. Of course he was only able to come to this conclusion cause he already felt that any life other than himself was an inferior being that doesn't deserve to live.

He just seems to be far more complex than people give him credit for...