Why does the author, William Golding, include Ralph, Piggy, and Samneric in the terrible crime of killing Simon in Lord of the Flies?

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.