When Humans reach the end, the explosion happens and kills anyone in the trigger zone, it works great. The problem is that some servers have respawn, and if a zombie manages to make it to the end as well, the other zombies are dying over and over again simulatenously, causes a lot of stress/lag to the player and the server. You might want to make the timer let up for about 5-10 seconds more and then kill them off (raise the damage amount, too, just in case), after the initial explosion. Note: This happens even when zombies have died and not at the same time, so you'll end up with one or zombie alive as the others die, and then have a loop of zombies respawning/dying, preventing the round from ending. Usually at that point I have to kill the humans in order to mediate the problem.