Q: What happens when I’ll Hold ‘Em Off is played when one player has zombies and a weapon, but another player has zombies, and the Sonic Tranquilizer?

…Does the second player’s Tranquilizer make it so that their zombie doesn’t count, and the first player wins?

A: Well, as it turns out, the zombie-nullifying effects of the Sonic Tranquilizer are optional. It is a device that can be turned off, rendering one’s zombies a threat again. So the logical assumption, gameplay-wise, is that is that the second player would “turn off” the tranquilizer so that their zombies definitely count, preventing the win. So one can’t really win with IHEO at all if anyone else has zombies, even if they have the Tranquilizer.

Thematically, it looks like this:
Despite wanting to survive the zombie apocalypse, the player with the Tranquilizer would secretly resent the other person trying to play the hero, and would refuse to let them face the zombies alone.

They would insist upon standing with them until the bitter end, instead of hiding behind the power of the Tranquilizer. (Oh, and preventing that player from winning the game, but really, it’s about supporting your friends through thick and thin, right?)