3. Seymour (Final Fantasy X)
Okay, I'm seriously starting to see a pattern of villains who decide that killing everyone is the only way to be saved. At a young age, he was traumatized with the death of his mother from losing her to Sin. Growing up, he thought that the best way to relieve everyone's suffering from Sin is to become it and destroy everyone so they wouldn't have to deal with it again.
In the end, Tidus promises Seymour that Sin will never return before Yuna finally sends the former's soul.