This is all interesting speculation, I've never thought about the charactization of Sephiroth like this. I've just went with the idea that Sephiroth went insane and perhaps even he didn't know his own motives before they were given to him by "his mother" who told him what to do. I also think it didn't help that Hojo started this whole thing in the first place with his experiment, putting cells of Jenova into Sephiroth was a dangerous thing to do, but at the same time, I think Hojo has been insane to begin with and therefore, he wanted his experiment in Sephiroth to suceed what no matter what, a more sane person would work to help Cloud stop Sephiroth, rather than aid Sephiroth as he's trying to destroy the Planet with an ancient forbidden magic.