Played 2 games of this tonight, both with 4 players.
In the first game, the colony started under a heavier-than-usual load of zombies. The main objective was to get every player armed (at least one survivor with a weapon for each player) and to stash away food. It was tight the whole game but everybody chipped in and we were all set to win on the very last turn... and then it turned out one of us was a traitor. On his turn, Sparky the dog sent one of his followers out to get people killed. Morale was right at the brink, and that put us over the edge. Traitor won.
The story was hilarious because that traitor was Sparky the Dog. Man's best friend engineered the downfall of our colony.
Caveat: checking back, I think he might have misread his objective slightly. I think he thought he had to have only one (armed) survivor, but he actually needed exactly one follower as well (as well as the leader). Even if that were the case, I'm pretty sure he could have easily won just by sabotaging the crisis on that last turn.
In the second game, locations were relatively light on zombies but the main objective sounded very difficult -- we needed to empty two item decks. Two of my starting character options were especially good at drawing many items... but I was the traitor. Instead, I took the Fireman (who was still somewhat useful to the objective, by potentially emptying an item pile of Outsiders cards) and a somewhat useless hanger-on (truck driver). My secret objective: have 4 barricades at a non-colony location, have 2 fuel in hand, and have a weapon attached to a character.
I immediately sent the fireman to the police station to look for weapons and fuel. I chose it over the Gas Station (which was about as good for weapons and fuel) because the latter did not have 3 spaces for barricades, which I would eventually need to build. I got my needed items pretty quickly.
But to avoid suspicion, I helped out a lot. Too much, I am certain. I actually chipped in heavily on early crises, even enabling us to gain Morale. I handed off many weapons to others. And on the second last turn, I actually helped to drain the Police Station item pile with 3 card draws. The Hospital was also almost empty, and could have been emptied that turn. However, several of the players still had the secret objective uncompleted, and we whined enough that the one who could have emptied the pile let us have a chance by leaving the last card at the Hospital untouched.
On the last turn, I killed the last zombie at the Police Station and build 4 barricades (partially helped by a Pad Lock I'd picked up early on), sabotaged the crisis (which players had given up on anyway; I just wanted to be certain) and wasted the rest of my actions on needless searches instead of doing something productive, blowing it off as just being ready for the game to end. And I was -- we still had 5 morale that I did not think we would lose.
But I had underestimated the danger of the crisis. It cost one morale immediately but also added a whopping 8 zombies to the colony, which filled up entrances and broke barricades. When we added the zombies from the regular end-of-round upkeep, it killed off enough characters to completely drain the morale.
These were the first times I've seen the traitor win. Previous 3 games were 1 victory (no traitor) and 2 total losses (both with traitor, also lost).
Great game.