Simon is an important character in the novel. He appears to be a little retarded, while others think him "batty". But his fits do not prevent him from being keenly insightful.
The circumstances of Simon's death are tragic and regrettable. He dies because of the timing of his re-emergence from the forest. Savagery has engulfed the boys and they are absolutely unable to design the beast; they do not care about their fellow man.
It is significant that by this death, the confirmation of Simon's earlier observation that the beast may be in the boys themselves will never be known. This death obliterates the light of civilization and insightful analysis. It can be seen that when Simon dies. hurting another severely no longer means anything to the tribe, if it had done so at all earlier;
When the argument at the assembly becomes a discussion of ghosts, Simon says that the beast may as well be the human being. This does not go down well with the rest. This is a point that might have been cleared had Simon not died.
Simon does see what the others have seen earlier. He does not run. He sees the strange sight for what it is and even without fear, reasons about dignity that the wind deprives the dead man of. So he relates the parachutist from his indignity in death. What is left is the revelation of what Simon has come to know. This knowledge would or might have restored sanity or prompted the savagery unleashed after Simon's murder. It appears as if his death opens the floodgates for further savagely and attack on decency. The boys have no qualms about further killing. The crushing of Piggy and the conch is closely linked with Simon's death in the sense that the questioning that Simon does is taken to a higher level by Piggy. At any rate the bespectacled "fatty" is the source of light and insight. Naturally, after the brave investigator of mysteries has been done in, the bringer of light follows as a matter of course.
With the death of Simon and Piggy, the silent antagonism between Jack and Ralph breaks into open confrontation and war as Jack and his tribe hunt Ralph.
It can be seen that the death of Simon is significant in several ways. By his death, the avenue to obtaining the key to the mystery of the beast is closed. In its place, the sluice gates for the unleashing of brutal attacks on civility are opened.