Flies have long been associated with the devil and his evil. "Beelzebub," a name for the devil, is Hebrew for "god of the fly," "host of the fly," or literally, "lord of the flies."
Stumblling through the brush upon the pig's head, Simon, the most intuitive of the boys on the island senses the evil indicated by the killing and decapitating of this pig. Jack and the other hunters, masked and muddy, have descended into savagery in their slaughter. Confronted by this Lord of the Flies, Simon imagines a conversation with it.
'Fancy thinking the Beast was something you could hunt and kill!' said the head....'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?....I'm warning you....We are going to have fun on this island! So don't try it on, ny poor misguided boy, or else....we shall do you...'
Simon realizes that the evil, symbolized by the pig's head on the stick with flies swarming around it, is the evil within the boys, the evil that Jack and Roger and the others have embraced as they reject the rationality of Piggy and sensibleness of Ralph. These resulting actions underscore William Golding's theme that without the restrictions and controls of an orderly society, human beings will regress into degenerate and savage behavior.
The title means Satan or the devil. (The pig's head is the devil, a representation of the evil within all the boys)