The pig's head surrounded by flies represents Beelezebub, whose name in Canaanite is "Lord of the Flies." He is one of the fallen angels, who destroys by means of tyrants, arouses desires, brings about jealousies and murders, and instigates wars. In Golding's allegory he is the manifestation of the evil that lies inherently in the boys. This is an evil that Jack has embraced fully.
When Simon goes to his secret place in the forest, he encounters the Lord of the Flies, whose
half-shut eyes were dim with the infinite cynicism of adult life. They assured Simon that everything was a bad business.
He talks to Simon and ridicules him by saying,
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 laughter shivered again.
The intuitive Simon perceives the evil, the savagery, that Jack has masked, but truly allows to rule him as he leads the others in anarchy, stealing the fire, and hunting Piggy and Ralph, and eventually killing Piggy. As he loses consciousness, Simon hears the Lord of the Flies tell him that they are going to "have fun on this island!" The evil inherent in man has gained control.
The head of the pig that Jack and his hunters put on a stake as a peace offering to the beast represents the evil that Golding felt was in all of mankind. It tells Simon, "I'm part of you.....I'm the reason why it's no go...why things are the way they are." Then it goes on to tell Simon that he isn't wanted because "...we are going to have fun on this island...". The beast, by echoing some of Jack's words and insinuations, reveals that Jack and it are basically the same. The beast laughs at and tries to belittle Simon for his logical and good ways. It proves that there is no separate, tangible beast; that the source of evil on the island isn't something that is an entitiy all to itself - it is the force inside of each boy.
In their conversation in the woods, the beast mocks Simon, telling him he ought to "run off and play with the others," since he won't respond. It goes on to suggest that the others will think he's crazy if he doesn't go out there and hang out with them. But it goes on to suggest that he always knew that the beast was part of them, not something outside or some kind of animal, etc.
At the end of their conversation, it appears to grow angry with him, some of its words mimic those of Jack, "we are going to ahve fun on this island," and it suggests that Simon isn't wanted, they don't want anyone figuring things out and trying to quell the evil within the boys.