This is an allegorical novel, and each of the characters and objects in it are symbolic. Simon represents the spirit present in each of us (in addition to Ralph who represents the physical, Piggy the intellectual, and Jack our inherent sin nature).
Given that, it's not surprising to see Simon as a kind of Christ figure throughout the novel. His death, then is a picture of Christ's death. He comes to them with good news but is killed for speaking it. His "broken body" is rolled out to sea, never to be seen (at least by them) again.
Most striking to me is the visual image of his body floating in the ocean:
"The water ...dressed Simon's coarse hair with brightness. The line of his cheek silvered and the turn of his shoulder became sculpted marble."
Then, the phosphorescent creatures of the ocean "busied themselves around his head.'' The image, of course, is that of a halo. His body, arms probably outstretched, is then carried by the waves out to sea.
This is a fitting end for Simon--he lived in a kind of moral isolation and he is buried, so to speak, in a physical isolation; he was bound by the evil on the island, and now he is free from such corruption and corosion.
Just think how horrible it would have been had Golding not written this scene the way he did. Imagine if he had left mutilated Simon's body on the beach to rot. The effect would have been almost unbearable to read. After the graphic, gruesome scene of Simon's murder, Golding changes the pace from frenzied activity to serenity. The sea creatures surround Simon's body, and it joins the rhythm of the waves. Simon is given the burial that he deserves. As the Christlike figure, the one closest to nature, the symbol of goodness, Simon returns to nature in death. Elsewhere in the novel, a more naturalist view is displayed of nature--nature is harsh, unyielding, oblivious to man's suffering. But in this scene, nature seems to be sympathetic, welcoming the one rejected by his fellowmen. This passage is beautifully written providing readers consolation for Simon's death.
From what little we know of Simon's life, he was in touch with a deeper sense of things than most of the boys. He was able to care for the little kids and interact with the big kids but he never became a part of either group or tribe during their conflicts. He also was able to discern through his periods of introspection what the beast really was.
In my mind, his washing out to see frees him from the evil and darkness of the island, which is fitting since he was the only one to really figure it out. He is washed away from the evil just as he managed to stay clear of it while he was amidst the boys.