Here I will do a calculation showing the probability of a player getting a Thriller medal (15 kills in a row as the Zombie without dying) in a custom game with 1 zombie and 15 humans. For simplicity we will assume that all players are of equal skill. There is no time limit on this game.
We will estimate that in a given encounter, the probability of a zombie killing a human is 0.28 (and hence the probability of a human killing a zombie is 1-0.28=0.72). We need a spree of 15. We know for a fact that the zombie will have a minimum spree of 1, so we don't need to worry about the first kill. However, in order to attain Thriller it is necessary that the next 14 encounters are won by the zombie. The probability of that is 0.28^14=1.8205912 × 10^-8. Next, we must calculate the probability that the first zombie got all of those kills. It's certain that he got the first kill. The probability he got the second kill is 1/2, since there are 2 zombies. The probability that he got the 3rd kill is 1/3, since there are 3 zombies. And so on. This calculation translates to one divided by fifteen factorial, or 1/(15!) because it will end with 15 players as zombies trying to get the last kill.
Since there are 16 players, your chance of being a zombie in the first place is 1/16 so we must multiply by this number. Therefore our final calculation is the following:
1.8205912*10^(-8)/15!/16=8.7*10^-22.
That's 0.00000000000000000000087, or 0.000000000000000000087% chance of occuring.
tldr: You should expect to get a thriller medal once every 1,149,400,000,000,000,000,000 games.
[Edited on 02.24.2012 9:58 PM PST]