More proof of how badly handled Sephiroth's change was.

This is his theme song once he turned evil in Nibelheim.
"Those Chosen By The Planet" fits how Sephiroth believed he was a Cetra destined to take the world back from the vile humans.

Only...he keeps this theme during the time of FFVII when he knows full well he's actually an alien abomination and directly opposed to the Planet.