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.