By default, battles are between a single zombie and a single hero at a time: zombies roll one six-sided dice, heroes roll two. The best die roll wins, and zombies win ties. If the hero wins and rolls a double, then the zombie is killed, otherwise the zombie is simply fended off.
So, let's get some probabilities down: the hero has a probability of 125/216 = 57.87% of winning, but only 15/216 = 6.95% of killing the zombie (15/125 = 12% of the time when the hero wins, or a little less than 1 out of every 8 wins). This sucks.
Occasionally, zombies get to roll 3 dice instead of 1 (with a particular combat card). This lowers the probability to 2183/7776 = 28.07% of the hero winning, with a 225/7776 = 2.89% chance of a kill (225/2183 = 10.31% of the time that the hero wins). This sucks even worse.
Rarely, though, though, the hero can win ties through a special ability or a condition on the card. Does this help at all in this last scenario? It pumps up the winning probability to 4109/7776 = 52.84% for our hero, and a 441/7776 = 5.67% chance of a kill (441/4109 = 10.73% of the wins). This is pretty comparable to the standard probabilities, although it reduces the probability of a kill to less than half of that in the initial probability. So it always makes sense for the zombie player to accept this scenario.
Overall? A bit of fun, though I probably wouldn't want to play it too often. Also, having two zombie players seems to severely disadvantage the heroes, since they play twice as many zombie cards. My group and I are thinking some house rules might remedy this.
The zombies actually don’t play twice as many cards – the zombies always only have 4 cards, if there are 2 players they are split between them. The odds for the game are always the same regardless of the number of players (X zombies and 4 heroes, dependent on scenario).
@Deniz: Oh, excellent point. We suspected that after playing it the wrong way. Thanks!
You should try some of the later scenarios (the mansion being a favourite) as these change how the game plays out. Less having to kill Zombies, more staying alive which makes the theme work well.