It might be realistic for kids, because kids aren't socialized as adults. In fact, we've deliberately delayed socializing them as adults more and more, and I think that's a grave error, which we've only been able to afford because we're so wealthy as a society. I think there are enormous costs, not only in the "educational" system that delays education more and more, but in the costs of that delay on the productive lifespan of people once they're finally accept as "adult", some 5-10 years after they were physiologically adult.
I'm reminded of a story Diamond(?) tells of two Polynesian(?) strangers meeting in the road. They start talking about common family or friends they might have. They are looking for an excuse not to kill each other.
Diamond is far from the only one to write about tribal killings in Papua New Guinea. That doesn't mean he didn't make it up, but if he did make something up it probably wouldn't be something easily proven wrong.
Are you sure? I am pretty sure there have been extensive studies on tribal groups all over the world that would disagree with you.
Also I think part of the success of Lord of the Flies was because it struck people as a realistic account of what would happen in that situation.