William Golding includes Ralph, Piggy, and Samneric in the killing of Simon to suggest that none of the boys are without inherent evil and guilt. Even though these four boys have stayed out of Jack's camp and represent a sense of good on the island, they must still deal with the evil that is around them. The four do nothing to help Simon, and their silence makes them just as guilty and responsible for Simon's death as the other boys. Lord of the Flies is meant to be an allegory that explores how the negative aspects of human nature affect us all, and no one is free from this element of human nature. We deal with it in various ways, but none of us are free from it. Golding highlights this idea in the novel by having these four boys present during Simon's death.
Simon is portrayed as a Christ figure. He is a prophet--he tells Ralph that he will "get back all right." He a messiah-- he tells the boys the truth about the literal and metaphorical beastie, that it is something inside of them. On the day that he is killed, he is trying to deliver another message--that the beastie is just a dead man on a parachute. And, like Jesus, Simon is good. He picks fruit for the little ones; he gives his share of meat to Piggy when Jack denies Piggy a portion; he helps Ralph build huts when noone else does.
As a Christ figure--the representation of good on the island, Simon's death shows the triumph of evil and ignorance over good. In order to show the pervasiveness of evil, Golding shows that all the boys either actively or passively participate in Simon's killing. In this way, Simon becomes a scapegoat for the sins of others.
When Simon sees the pig's head on a stick, he has a vision. The pig's head tells him that "we want to have fun on this island." He cautions Simon that he should "play" as the others do, or they will "do him." In this case, "play" means turn savage. This scene is equivalent to the temptation of Jesus in the wilderness. Simon is tempted to become savage, but he cannot. He is inherently good.
Thus, he is killed. Each boy has evil within--even Ralph, even Piggy, even Sam and Eric, and this evil manifested itself in the killing of Simon.