I'm with Khush--it's not fair to throw a curveball this far into the game.
Let's say players A, B and C were working together. Player A got together with players B and C because of voting records, hunches, investigations, whatever. Suddenly one of those two could be the killer? There's no way for player A to know who he can trust without starting over from square one. He can't use past voting records as an indicator because they mean nothing going forward. He can't trust any past conversations investigations because someone out there suddenly changed alliances. And they didn't even have any say in it! Not only are you hurting the players who will be killed by the killer, you're hurting the killer, too--he's gone from a position where he could likely win the game to a role where he's almost certain to lose and in no way is it his own fault.
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them? To die: to sleep;
No more