As Ralph stares at the skull, picked clean by the flies, he is filled with a "sick fear," partially connected to the earlier questions in his mind about whether or not the other boys killed Piggy on purpose. Ralph refuses to accept it, at least on a certain level, and keeps telling himself that it is an accident.
But the beast, or the head on the stick, leers at him, suggesting that it knows the answer that he is too afraid to admit to, that the boys really are the beast and he should be terribly afraid of them. As he lashes out at the beast, it then grins at him with an even wider grin after it is split on the rock. But it still wont give him the answer, he has to come to it himself.
"The skull regarded Ralph like one who knows all the answers but won’t tell. A sick fear and rage swept him. Fiercely he hit out at the filthy thing in front of him that bobbed like a toy and came back, still grinning into his face, so that he lashed and cried out in loathing. Then he was licking his bruised knuckles and looking at the bare stick, while the skull lay in two pieces, its grin now six feet across."
When Ralph stumbles upon the boar's head he doesn't know what it is at first. When he realizes what it is he knows it is important somehow, but he can't figure out how. He feels the head is "grinning" at him and is part of the reason things have "gone wrong." Like Satan, he knows the answers but won't tell us. Satan drives mankind to behave in evil manners but we don't know why. Ralph tries to destroy this evil, by destroying the boars head, but takes the stick it is mounted on to fight Jack, Roger, and the tribe.
Ralph, unlike Simon, has not figured out that he is fighting not only Jack, but the evil inside all mankind. Simon had figured that out when he had his "conversation" with the pig's head. The head told Simon "I'm part of you." Since Simon's conversation was part of an epileptic hallucination, the realization that the evil is inside of man must have come from Simon himself. Unfortunately Ralph, has been so focused on trying to lead the boys, thinking they would follow him simply because he was chief. He never quite realizes that there is an evil in the boys, as well as himself, that must be overcome with strong government and laws. Ralph's leadership, based simply on one vote and no real authoritative actions, has not been strong enough to lead the boys or to get the head to tell him the answers. Ironically, Simon, who did know the answers, is dead, killed when Ralph himself was taking part in the ritual dance.