By having the children at war on the island, Golding is saying that people are, by their very nature, savage. He is showing us that these kids will lose their civilized veneer when they are dropped on the island and have no one to force them to behave in civilized ways. So this implies that we only behave in civilized ways when we are forced to (by the pressure of our society if not by actual violent force).
But, at this point, the adults are at war too. This tells us that even the civilized world is not all that deeply civilized. It tells us that all humans are pretty much ready to fight each other and revert to savagery at any time.
So by having both groups at war, Golding is saying that civilization keeps us out of savagery, but that even civilization is not all that strong -- we are all savages beneath the skin (and not very far beneath at that).