Again, you seems oblivious about the situation there, where it's pretty much the Wild West all over again, in which it's kill or be killed, simple as that.
The Police group were lucky they had the MC to solve all their shits, because without him, they're already dead by the other group, since they're trusting people way too much.
lol, you called the MC's act as "barbarism", despite it's the most logical thing to do, where there's absolutely no merit in keeping the guy alive, and it's just a complete waste of resources, since who knows what he could do next, considering he doesn't have any ties with the group.
The problem right now is that you're still seeing those survivors from a complete different point of view, in which you still see them living in a world of peace and order without any conflicts, while the world that they were living is pretty much a desolate lawless land that were overrun by the living dead, so it doesn't make any sense at all.
Like, you can literally simulate living in that kind of world by playing survival online games like Project Zomboid, DayZ, Rust, etc, and see how very different people were taking matters there, despite it's only a game.