Sephiroth finds out a dark secret about himself, which drives him over the edge into insanity, but after that there is very little talk of his motives, it becomes "I want this! I want that!" but nothing else.
yep. also, that's a crap reason to go insane about "People nearly wipped out my ansestors 2000 years ago. i'm going to get myrevenge now."
Please feel free to read my take on the official novalisation of Dissidia Final Fantasy at this link: