Well, the first game implies pretty heavily he's not trying to just "do FF7 again but win", but is instead trying to make sure he and Cloud can do the murder soulmate thing forever instead of allowing someone/thing else to "end everything" The second game has less of this but also makes it obvious he's actively helping the party at multiple points in order to do something weird with the "baby's first multiverse" the events of the first game created, and while Aerith seemingly pulled one over on him by the end he's still in a winning position, whatever that fucking means. So tl;dr Sephiroth is trying to do something in the background that he either can't or doesn't want to pull off without Cloud on his side, and the rest of this is either trying to manipulate Cloud to that end or the multiverse equivalent of RNG manipulation The games are just being really obtuse about it