SC2Mapster

I agree with UnluckyClover. Stay faithful to the 1982 movie version of The Thing. Make the Thing really overpowered and the players who aren't the Thing feel almost desperate in their attempts to stop it. Maybe even add more than one way for the Thing to win. For instance, instead of just killing everyone who isn't infected, make there be a possibility for the Thing to escape the quarantined zone via transportation, like say a helicopter or boat, or something. Or maybe the Thing can destroy the power generator and kill everyone off by freezing them to death. The players will be forced to work together to guard certain areas, instead of just bunkering down somewhere and waiting till the time limit is up. And that's when the psychological aspect kicks in. Forced to work together, but who can you trust?

A secret to the Starcraft 1 that made people come back to it over and over is that it is a lot easier to win if you're the thing/ hard to win if you are not. Better to make this game too hard(for clean humans) than too easy.