Simon's conversation with the sow's head on a stick is important. Simon is learning clues as to what is going on around him. The sow gives clues as to the dangerous character traits found within Jack and his hunters. The sow indicates that the evil is within the ones who are stranded on the island.
Simon had thought this earlier and even spoken that the beast lives within the boys.
The conversation with the sow's head is just confirmation for what Simon had already suggested. The evil is within. Jack and his hunters are only responding out of an innate evil, an evil so close, it overtakes them:
Simon imagines that the head replies: “Fancy thinking the Beast was something you could hunt and kill! . . . 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?”
The author is just relating that man was born with an evil that will surface in the right circumstances. Innocent boys lose their innocence as evil prevails. The boys were destined to be evil. It is a unique part of nature that is found within mankind or in this case within the boys. There is no escaping the evil within man. It is a part of human nature. Becoming stranded on the island brings out the evil nature in Jack and his hunters.
Simon is quite intelligent. He recognizes that the evil is innate. He is worried about the dangers involved in Jack and his hunter's actions. Simon was all too aware of the dangers that could result from Jack and his hunters. As they kill Simon, the reader learns that Simon was right.