My personal interpretation of the game was that Jenova had poisoned and manipulated Sephiroth and that by the end of the game even with Jenova destroyed/killed Sephiroth was still carrying out her will (because he pretty much was), but the Ultimatania guide pretty much reverse the who's who in my interpretation.

I think that ultimately looking for deep meaning in why Sephiroth was doing what he was doing is the wrong way to go about looking at this game. Sephiroth went nuts and I think that should be the ends end of why he started trying to kill everyone, his goals are ultimately unimportant, he's just a crazy dude.