Sephiroth is the best villain of all time. However there seems to be some debate over whether he is a villain or not. I think this debate comes from the fact that he is really cool and no one wants to think of him as a bad guy. So they say Jenova was really manipulating him. Here are some reasons why I think he was insane but not manipulated. (Its up to you whether you think mental illness is an excuse.)

1. The Jenova part in the crater calls him Lord Sephiroth.

2. He fights you after you frag Jenova at the end.

3. "One winged angel" does not say "do not let me die". Check out the mistranslation page.

4.He died five years before the game started. So he no longer has a body for the Jenova cells to be in. What you fight in the game is his ghost.

   Either way he is still a kick as villain!