Pumpkin raises an interesting point... Though it would depend on how that well twist can be played. I feel a reveal of that magnitude about the main protagonist(s) generates a need for a degree of foreshadowing. Pulling it completely out of the dark that you'd secretly been the real evil all along could go rather poorly.

One idea that strikes me for making that reveal have a real kick to it, perhaps certain members of the party happen to be Unwitting Pawns? Pawns the player will be required to personally dispose of once they either figure things out or are of no more use.