The above answer makes some good points. The boys on the island are not faring well and as the book progresses they will fare even worse. The base desires of their hearts will break down any semblance of order and civilization. Anarchy will rule the day. In light of this, the reader might think that what the boys need is adult supervision and intervention - adults will know better, right?
When the dead parachutist comes down, it is a sign from the world of the adults - a message from heaven, in a sense. But the message is not hopeful or helpful. In other words, if the boys need a role model or help from the outside world, they are in trouble. The war overhead (literally) shows that the adult world has problems as well.
Also the fact that the man is dead shows that the adult world cannot do much. To tie this passage into the theme of the beast, if the beast is within, then the beast is also within the adult world - a bleak note indeed.