Simon fights with all his feeble power against the message
of the head, against the "ancient, inescapable recognition." The recognition against which he struggles is the revelation to him of human capacities for evil and the superficial nature of human moral systems. It is the knowledge of the end of innocence, for  which Ralph is to weep at the close of the book. The pig's head seems to threaten Simon with death and reveals that it is "the Beast." 

" 'Fancy thinking the Beast was something you could hunt and
kill!' said the head. For a moment or two the forest and all the other dimly appreciated places echoed the parody of laughter. '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?' "