The parachutist's body, which sways back and forth because of how the parachute strings are caught, is a physical representation of the beast. Therefore, it represents fear and violence. It has also become somewhat like the conch in the fact that Jack uses the presence of the beast as a tool to frighten the others into succumbing to his authority. As time goes on and more boys join Jack's group, there is a shift in the boys' sense of importance. Their fear of the beast (parachutist) and all it represents eventually outweigh their desire to be rescued (keeping the fire going). Preoccupied with the beast, they descend further from civilization, further into savagery.
Like the plane crash that places the boys on the island in Lord of the Flies, the dead pilot is a result of a global conflict, a man lost far from home. In the context of the story, the dead pilot can be interpreted to symbolize several ideas, the primary one being the idea of fear.
The boys have debated the presence of a beast on the island before the pilot falls to the earth. When the dead man is discovered he is seen as the beast.
The boys' misinterpretation of the dead man as a beast is only a partial misinterpretation, however, as the dead man is certainly an indication that the boys' fears have foundation. The threat of violence, embodied symbolically in the idea of the beast, is proven to be real in the presence of the dead pilot.
Though the pilot is not a monster, he is related to a world of extreme violence; a world of war. Symbolically then, the dead pilot represents both real violence and the boys' psychological fixation on the idea of danger.
More simply, the dead pilot brings death once again to the island. The boy with the birthmark dies early on in the story, before the boys really know what's happening. The pilot marks the end of the unreality of death. Ironically, the pilot is both the fulfillment of a fantasy (the beast) and the end of that fantasy (death and violence become real; Piggy is killed; Ralph is hunted).