My reasoning's for not inclusion is not because he didn't have a motive. As stated above, his motive was a twisted revenge against humanity but in reality his true motive eventually became fulfilling his "mother's" will. Jenova was a planet parasite (I'm going to ignore that this is a complete ripoff of Lavos from CT ahem...) Sephiroth was going to basically fulfill what Jenova couldn't since Jenova is a hollow fragment of her former self by the time of FFVII. It's not a bad premise it's just that ultimately Sephiroth is just another pawn. He thinks he's doing it for himself but he's only really fulfilling Jenova's basic nature. This kinda've makes him something to be pitied in my eyes.

Kefka was just a narrcissistic(sp?) sociopath when it comes down to it. Sure there's the little backstory about the failed magitech knight experiment but we never really got to see Kefka before that. For all we know, he was an arrogant jackass to begin with; and the experiment just finally pushed him over the edge.

Kuja is very similiar to Kefka but we at least get to hear more about his back story and it comes off more convincingly.