Yes, the parachuist and his ruin is a metaphor for the spiritual ruin of man. Innately evil, man is further corrupted by wars and civilization is in ruin as a result. His tattered and torn body is comparable to the spiritual destruction of the soul of man.
The most obvious extended metaphor in chapter eight is the Lord of the Flies. Golding uses the symbolic ruin of the fly-bloated sow's head to represent decay, corruption, and evil-personified in the Lord of the Flies. Simon's conversation with the Lord of the Flies reveals that the true evil on the island lurks within the boys themselves.
One metaphor is the beast. The beast is a running extended metaphor throughout the book. The boys fear the beast, but are not sure if it exists. The beast is a manifestation of the evil that exists inside them. Many refuse to accept this evil, just as many boys refuse to accept the beast.