The parachutist also gives the boys' fear of the beast more credence. They are afraid of the parachutist as something they can't explain. Consequently, everything they can't explain becomes known collectively as the "beastie" which Jack uses to manipulate the boys to do what he wants them to do.
The irony is that while the boys on the island struggle to build a society, the adults in the world the boys were fleeing are killing society. Also, the boys on the island, particularly those under Jack's leadership, are becoming increasingly less civilized and more brutal. The parachutist, a victim of war, represents the outside world and bringing proof of the Lord of the Flies' allegation that mankind is inherently evil; that each person has an evilness within himself. Golding felt that war was evil and uncivilized (his experience in the war probably led him to feel this way), so he poses the image of civilization (technology that produced the aircraft and thus the parachutist) against the incivility of warfare (the parachutist is dead because he was shot down by the enemy).
What is more ironic is that the parachutist is a sign to the boys. Considering the fact that the parachutist is a shot down by the enemy, making this evil, can it truly proven the fact that mankind is evil. What I mean by this is that they get a sign by evil. Not only this but at the end of the story they are saved by a military man in the middle of war. So does this mean that they are saved by evil? All because people believe that war is evil. I haven't decided. :)