Towards the end of chapter nine, the boys are dancing on the beach, reenacting the killing of the pig. They are chanting "Kill the beast! Cut his throat! Spill his blood!" They have let themselves become savages and released their moral obligations because they no longer want the burden of thinking about their actions. They have embraced the sadistic side of human nature. When Simon stumbles in to the circle, the boys are frightened at first and attack him. In the confusion, their fright turns vicious and they submit to the beast within themselves.
Simon's death is ironic because he has come to tell the boys that there is hope of being rescued because "the beastie" was nothing more than a parachutist. However, the brutality of the group has more power than the truth, and Simon is not able to deliver the information.