Listed below are several theories regarding the nature of Sephiroth: his motives, his end goals, and some rebuttals against those who claim he is nothing more beyond an antagonist. Be prepard for spoilers.

The Softer Side In defense of Sephiroth; why he isn't always as bad as he seems.
World Without Sephiroth Theory: is Final Fantasy: Advent Children's Sephiroth really the same Sephiroth of Final Fantasy Seven?
Jenova : The True Enemy Because mothers can be a bitch.