Well, for example you walk under a roof and the Zombie Master release the roof and it kills you, if you are skilled you can avoid it. Also there is this trap...a container standing half over a cliff that leads down into the abyss.
In this container, as far into it as you can get there is a shotgun and some ammuntion. Will you run into the container in hope that you will get your hands on the shotgun and that the Zombie Master wont push it over the cliff?
Releasing traps and spawning zombies requires resources which the Zombie Master has.
So to kill one single survivor isn't always maybe worth it.