In Chapter 8, the Lord of the Flies tells Simon that they are going to have fun and not to interfere. What will happen to Simon if he does interfere?
“—Or else,” said the Lord of the Flies, “we shall do you? See? Jack and
Roger and Maurice and Robert and Bill and Piggy and Ralph. Do you.
This is, understandably, a rather vague statement on its own. In order to get a better sense of what the Lord of the Flies means, it is best to look at the passage-- and Simon's role in the novel-- as a whole. Simon was in the choir, but was decidedly diffferent from the other choir members, who became the hunters. [The Lord of the Flies calls him "queer," and readers must resist the urge to make the connection to slang for homosexuality.] In this mini-society, anyone seen as "different" can become a target. The Lord of the Flies is reiterating this by saying everyone else "fits in" to the society, while Simon remains different. If he doesn't make an effort to fit in, he will be removed from society. The others will do to him what they have done to the pig from which the Lord of the Flies is speaking.