What caused the boys to act the way they did during the game in the lord of the flies?

Most people would probably pinpoint the death of Simon as the most emotional event which occurs in the book. The aftermath of it leaves Ralph in a state of shock, as he realises the depths to which the boys, and indeed himself, have sunk. Piggy tries to treat the matter logically and practically, it's done and can't be undone and to lessen the impact he attempts to lay some of the blame on to Simon himself. Samneric try to pretend that it never happened, they are in self denial. Jack simply dismisses it as a 'trick' of the beast, something intellectually intriguing but of little or no significance.