I read a long time ago on a website that had a theory that Sephiroth was dead and or was actually simply being controlled by the monster Jenova (sorta like in Final Fantasy 4). Jenova was using him as a manner to hurt the planet so that when it tried to heal it's self, it could absorb the energy and destroy everything. I tend to believe in this theory cause it makes sense to make and at the end of the game, the first Sephiroth form has things over it's head like puppet strings.