In many ways, Simon's fate in the novel "Lord of the Flies"by William Golding is a self-fulfilling prophecy. If the story played itself out without rescue, then probably this applies to every boy on the island as it would have become a case of "the survival of the fittest." The weakest, youngest boys would have been picked off first for sport, but the thirst for bloodthirsty past-times would not have gone away. The beast of uncontrolled impulses would have won out and the most powerful boys would have got bored and wanted more. Piggy and Simon were just the first of a long line of deaths. Even if Simon could change his character, it would not have saved him for long - it would have been boy-kill-boy til they were all gone. Luckily, they were rescued.
In a more symbolic sense, the pig's head represents evil incarnate, temptation. We know that from Golding's notes on the novel in which he equates "Lord of the Flies" to Beelzebub. On the other hand Simon has striking similarities to Jesus. He is kind, compassionate, intuitive, prophetic, and messianic. The pig's head acts as his tempter, equivalent to Satan's temptation of Jesus in the wilderness. What Simon is being asked to do is "have fun on the island." This phrase is most often used by Jack, and to him "having fun" means hunting and killing. Almost every other boy on the island with the exception of Piggy enjoys the savage nature of the hunt. Even Ralph enjoys participating in the boar hunt, and later engages in the pig-hunt reenactment, which almost goes too far--Robert acting as the pig is almost hurt by the boys surrounding him.
Simon is asked to join in the fun or savagery. In order to survive on the island, he must change. The consequence for remaining civilized is death. As the pig's head tells Simon, the boys "will do him" if he does not "play."
It is not Simon's nature to change, to become savage, to hurt, or to kill, and he resist this temptation by falling into a fit. When he comes to, he goes to tell the boys the true nature of the beast. This action leads to the fulfillment of the pig's head's prophecy. The boys do indeed "do him."
This quote is spoken by the pig's head (the Lord of the Flies) to Simon right at the end of the chapter. To me, it is basically a warning to Simon that the savages are going to win out and that he, Simon, had better not try to stop them.
Simon is the person who has come closest to understanding what is going on on the island. He has tried to warn the other boys, but they have ignored him. The head is telling him that he had better stop trying to oppose them. As it turns out, the head is correct and Simon really should have feared the hunters...