Who kills Simon and Piggy in Lord of the Flies?

Simon's death was largely the result of mistaken identity by all of the boys, during an overexited pig dance which happened in an evening thunderstorm. The description of Simon's death conveys the almost animal ferocity of the attack by the boys. Piggy's death in constrast was the direct consequence of the action of one boy, Roger who, in full daylight and aware of the possible consequences, just went ahead and launched a boulder from the top of castle rock simply because he wanted to.