A traitor. Seeing as how there are huge overtones of trust and loyalty stressed throughout the entire course of the game. It struck me as weird that this wasn’t a feature, most of the game-play is just recruiting new members (you hardly know) and completing loyalty missions. Not to mention in Mass Effect 2 you are working for an organization that you don’t trust. It would be an interesting sub-plot to venture into, giving the player even more incentive to help other characters find closure.
EDIT: Treachery sounds reasonable considering the enemy has the ability to control people’s minds. So in order to fully cleanse someone, you would have to aim for extreme loyalty.