Simon's death is one of the greatest tragedies in William Golding's Lord of the Flies, both because of who he is and how he dies. Simon is the character who is most sensitive and represents the best part of human nature. He is the only boy who recognizes the true beast on the island--it is them. When he frees the parachutist, the beast from the air, he is demonstrating a compassion which he is not given. In these final paragraphs, it seems that Golding is mourning the loss of civilized behavior.
When Simon meets the Lord of the Flies, he is forced to recognize the reality that it is the nature of man which is causing such savagery on the island; when he tries to tell the others about his discovery, they kill him. It is true that they did not quite realize what they were doing in the midst of a dark, stormy night and in the middle of a chanting, frenzied circle; nevertheless, the boys did kill the lone voice of truth.
Because truth and goodness are now gone, Golding chooses to mourn that loss in the last four paragraphs of chapter nine. As the answer above suggests, the connection of Simon to nature is clear, as is the Christ imagery.
Nature provides him with a natural halo:
The water rose farther and dressed Simon’s coarse hair with brightness. The line of his cheek silvered and the turn of his shoulder became sculptured marble. The strange attendant creatures, with their ﬁery eyes and trailing vapors, busied themselves round his head.
Golding stops short of depicting Simon with his arms spread out like Christ on the cross; however, the image is not hard to imagine as his body floats lifelessly out to sea.
The great wave of the tide moved farther along the island and the water lifted. Softly, surrounded by a fringe of inquisitive bright creatures, itself a silver shape beneath the steadfast constellations, Simon’s dead body moved out toward the open sea.
As Simon's body floats away, so does the best part of human nature (symbolically). What happens after this is a quick descent into savagery, for there are no elements of nobility left to stop it.