"a sadistic, fiendishly intelligent lunatic with a warped sense of humor, deriving pleasure from inflicting twisted, morbid death and terror upon innocent people"

"he is a textbook example of antiscocial personality disorder"

"symbolized as a killer clown, driven by a disordered mind to pursue destruction and chaos with as much panache as possible"

"he has been willing (and eager) to wreak as much havoc as possible upon innocent people"

That's from the Wiki description of The Joker from Batman, but it applies pretty well to Kefka IMO.

That's not to say that Kefka is FF version of The Joker, but The Joker was certainly one of the big inspirations for the character.

Whereas The Joker was rendered insane by the physical trauma caused by his chemical bath, my guess would be that Kefka went insane with the botched magitech infusion. His ties to The Joker character would suggest that Kefka is "fiendishly intelligent" though with a "disordered mind."

Basically, I envision him to have been a either a super-soldier/general or a wannabe, immensely proud and independent and with no concern for commoners or subordinates, who, rather than having any intense loyalty to the Emperor, saw the Empire as by far the best opportunity to prove his abilities...to himself.

Being a person who gained satisfaction and motivation solely from his own success, pre-lunatic Kefka shared a lot in common with many high-profile athletes...and like them looked for any advantage to "make himself better." Whereas a professional athlete might have resorted to steroids to boost himself from "hall-of-famer" to "best-ever," Kefka willingly joins the experimental magitech program out of a desire to view himself as the best solider/general/military mind in the world.

Then of course the "accident." His intelligence and his intense drive to consider himself "the best" are still there, but perhaps uncontrollable or unrecognizable even by himself. Those traits are also filtered, or perhaps used, by the chaotic, homicidal part of his personality that arose from his "deformation."

As to why he submits to the Emperor until his power grew incomprehensibly great on the floating continent, Kefka's quest for power is not about ruling land or people. The Empire had the resources Kefka wanted, but I think the power over land and people that the Emperor specifically had would only be by-blows of Kefka's true desire--to simply be as "powerful a being" as he can in terms of intelligence, prowess, and magical power (on his more rational side) and in causing death and destruction (on his lunatic side).

An evil ruler like Gestahl kills because while he loves power he actually lives in fear of losing his power at the hands of those who oppose him. But Kefka wasn't afraid of losing his power...he killed because he could, and because he enjoyed it.