This is a great question. The death of the parachutist is an intrusion of sorts into the world of the boys. It reminds the reader that there is a context to the novel, The Lord of the Flies. The context is one of war. From what we can tell the war that is going on is terrible. In fact, all we know from what Golding states is that atom bombs were a real possibility against England.
The war is also the reason why the plane of the boys had to crash land. In light of all of this, the adult world is not much better. To put it another way, war threatens civilization. This is also the chapter where the order of the boy is threatened as well.
In short, the parachutist and the on going war reminds us that all of humanity struggles with order and being civil. The world of the boys, then, is a microcosm of the world of adults. Why should the boys not be at each other's throats? Is this not what adults are doing?