The beast and the lord of the flies are both symbols of man's savage instincts and evil itself. The dead parachutist does not seem to suggest these dark connotations, but perhaps the distinction is not as defined as it seems.
On one level, the parachutist might symbolize the civilization from which the boys have been cut off. He comes from the world outside, the world wherein they used to live before becoming isolated on the island. He might be interpreted as a symbol of the world as a place of sophisticated technology and progress far removed from the primitive conditions of the island. The parachutist is an adult, perhaps symbolizing order, mastery, and authority in contrast to the boys' immaturity. He may be seen, therefore, as symbolizing all the boys have lost. Simon seems to view the parachutist in these terms as he attends to his body with gentleness and respect.
On a deeper level, perhaps the parachutist symbolizes the essence of the beast and the lord of the flies, savagery and evil in action. His remains are the result of battle; his body is a product of warfare. The war raging in the outside world is rooted in the same primal instincts that have created war on the island. Civilization exists nowhere, and no one is safe from what lies within.