Lord of the flies yes, at the end of the day in a world without rules, law enforcement, hell even the basics people get nasty, take a look around in any disaster where relief has not reached the area in under two weeks and you’ll arleady see the cracks forming.
as for trusting groups of other survivors, you have to ask:
who are they?
what do they want?
are the crazy?
are they part of the gangs we ran into?
are the diseased?
hell you can add to the list, but in that kind of environment where the only help and support you have is yourself those you know, along with whatever you can scrounge together, your not going to simply open the door after a five minute chat and let them in, un-searched or unchecked. harsh as it may sound, that’s a fools mission. worst still allow them in as a group instead of one at a time so as to be sure.
well that’s what happened, luckily for them they turned out to be the real deal, and one of them was armed, though he did admit to carrying a knife and surrendered it when questioned.