Typical of manga protagonists short term thinking. Next time cannibals will have bows too.... or throwing spears, or will come for them at night. They should finish them all now while they have advantage.
Incredibly frustrating to watch, but probably kinda realistic, both WW has proven most humans just can't bring themselves to consciously pull the trigger to kill another human, even if it means saving their own life.
Now I WANT the protagonist and his friends to suffer. How naive and short minded do you have to be to just leave them alone, when you have the upper hand? I get killing a human is hard, but not even for your friends?