So far, most of the problems in this show have been caused by stupidity. Carelessness and complacence, and just straight up stupid decisions.
The other problems have been caused by assholes directly and intentionally causing problems. First that asshole boss who wanted to control people and hoard resources, and now this batshit group who just wants to cause chaos.
I realize this happens in the real world too, but it's still really annoying.
At the very least, Akira and Co. already made various mistakes and saw the mistakes of others. They have experience, and should've had those trapped zombies killed. Even if they didn't expect those weirdos to cause problems, there's no sense leaving a bunch of zombies around.