It's been confirmed that the one of the last night kills was made by a rival.
5 murders = 3 mobsters, 1 vigilante, 1 rival.
If the rival goal is only to get rid of the mobsters then effectively he'd only be a second vigilante.
Rival MemberThis means that if the mobsters are all taken cared of and there are more then three players left, the rival has to continue killing until the number drops to that level.
One member of the game will be a Rival to the Mob. He is like a townie but has the ability to kill once every three nights. He wins by being the in the final three.