The LotR reference is perfect!
And remember: One ring to bring them all and in the darkness bind them.

Everyone with Jenova cells, Cloud included, were lead to The Reunion. Thus, Sephiroth was still somewhat Jenova, and planning to combine all the cells back together, because as Ilfana (or someone idk) said that Jenova will keep trying to repiecce itself back together, as it did back with the Ancients, and as Ilfana predicted.
IMO Sephs became a successor to Jenova, but she was still alive inside him. He did take a turn and change his mind about things, but I feel that was Jenova's influence.