Kefka - FFVI

Not only is he a mentally unstable misanthropic nihilist but literally destroys the world mid-way through the game in order to become its god. By the time you fight him in his tower built from the rubble of a destroyed world he has turned into a twisted angelic being.

Sepiroth has nothing against this guy. Frankly no other video game villain I have ever encountered is remotely as epic.