Oh...well...carry on then.
Well...I mean....once Sephiroth figured out that JENOVA was KIND of his mom, and kind of this weird statue thing being hauled up in this power plant that is suspiciously now infested with mutants, he becomes obsessed with finding out who/what he REALLY is. Then he kind of finds out he was created through a series of genetic experiments. Not only that, but he was essentially created from an alien which, that and of itself, could probably mess you up pretty bad (albeit he didn't KNOW she was an alien right away). Essentially it's pretty similar to kefka with a twist. In retrospect, a pretty over the top twist.
Then he finds out that JENOVA's real purpose was to destory everything on the earth, he decides to follow in mom's footsteps. BUT pretty much Sephiroth, like Kefka and Cloud, is pretty much a ticking time-bomb of bananas.