By using it then, you sealed your fate on the next (last) day.
Even today, using the smily rule (a subpotimal rule which is fairly easy for the town to guess), you almost survived, even though everyone knew you were the last mafia. At that point, there may have been more less doubt about it than today.
I maintain that we should have kept the Zack rule until we reached 2 vs. 1, then used it to ensure victory.