With the murder of Simon in William Golding's Lord of the Flies, what is completely stripped away from the boys and the island?

The murder of Simon holds many keys to the fate of the boys on the island, however none more important than with the death of Simon, the fate of the boys will be sealed. Since Simon alone knows the truth about the "beast" because of his conversation with the Lord of the Flies, and he knows the moral truth- as he dies so does the truth. Thus the boys are not able to recieve his revelation or message when he is killed by the frenzy of the hunters.

The truth he posesses is that the beast lives in all of the boys, and in all of mankind. It is the evil heart of corruption and barbaric savagery that enticed the hunters. It is this seduction into darkness that was too much for the hunters to ignore or escape from- especially with the absence of laws upheld by the adults of the cvilized society.

It is also a good idea to look at jseligmann, also a teacher online here. In his Q&A a few weeks back he gave the examples that also support this claim. According to his answer and Golding's novel,

"Simon has found the head of the recently and savagely slaughtered pig in the forest. Simon sits transfixed. The head, with the indifferent, sated flies buzzing about it is a testament to fear and survival and to our all-too-human propensity for gore, pleasure and violence.

'At last Simon gave up and looked back; saw the white teeth and dim eyes, the blood—and his gaze was held by that ancient, inescapable recognition.'

Yes: ancient as life and inescapable as violence and death.

“Fancy thinking the Beast was something you could hunt and kill!”

So says the Lord of the Flies. How can one kill what is part of one's own nature? Might as well try to kill your thirst or need to sleep or to breathe or breed.

“You knew, didn’t you? I’m part of you? Close, close, close! I’m the reason why it’s no go? Why things are what they are?”

It is the heart of darkness deep in our own beings."

This was the message that simon was bringing down from the mountain. His information that would open their eyes and see the evil of their ways. Essentially the hunters seal their own fate with his death, and unfortunately help the prophecy of the Lord of the Flies come full circle.