My guess is Cid is the last mafia. Remember when Faris said she 'tracked' Cid and the target was Faris? Yeah, only one way for her to be 100% sure she wouldn't be outed by that little claim...

It puts Cid on the non-Poisoner team, too, which means he could have easily be poisoned earlier in the game. Should have let him die, Bleys!