I believe the term for this is Villain Protagonist, coupled with Hero Antagonist in this context. Thusfar it has been a rare approach to games that I rather enjoy. While I know the crossover based Dissidia games had the antagonists as playable, to my knowledge it lacked the option of an 'evil campaign' that makes a true Villain's Game. Hmmm... so many possibilities for this with Final Fantasy....

I have to agree with Ayen that a playable Sin would be so much fun.... Lets start with Bevelle, oh fiend-whale of genocide. Or perhaps tear some new orifices into the time-space continuum with Ultimecia.