how does the dialogue of Simon with the severed head of the pig show the theme that humans can become evil in the absence of civilization?

Simon comes face to face with the pig's head on a stick. It is the Lord of the Flies or manifestation of Beelzebub (Devil). The Lord of the Flies speaks to Simon in the voice of a Headmaster. It is an extension of the dark side that Golding thinks exists in all of us. Only Simon has the faculties to find it. Simon is Golding's Christ figure and he understands that the evil in the world is inherently inside the boys themselves and that he must "save" them.