There are actually three deaths in William Golding's novel Lord of the Flies, but the two you mention can be classified as murders and are therefor more egregious. The first death is the little boy with the mulberry birthmark who is inadvertently killed in an out-of-control fire in chapter two; Simon's death borders on accidental but Piggy's death is flagrant murder.
Simon is killed on a dark, stormy night after he has discovered the truth about the beast from the Lord of the Flies. He is weak and exhausted, but he wants to tell the others that the beast is in all of them, is part of them. He crawls through the dense foliage to get to the spot on the mountain where the boys have all gathered to celebrate a successful hunt and eat meat.
The boys have all gathered into a circle and begin to chant the same words as they do on a hunt: “Kill the beast! Cut his throat! Spill his blood! Do him in!” Some of the littluns assume the role of a pig and a kind of a mock hunt ensues. The lightning, thunder, and rain are crashing around them, and it is clear the boys who are chanting in a circle have gotten caught up in the atmosphere and emotion of their setting--and then Simon appears, crawling out of the woods in the dark. He crawls to the center of the circle and tries to talk to them, but his words are unintelligible to them and they do not listen.
The sticks fell and the mouth of the new circle crunched and screamed. The beast was on its knees in the center, its arms folded over its face. It was crying out against the abominable noise something about a body on the hill. The beast struggled forward, broke the ring and fell over the steep edge of the rock to the sand by the water. At once the crowd surged after it, poured down the rock, leapt on to the beast, screamed, struck, bit, tore. There were no words, and no movements but the tearing of teeth and claws.... Presently the heap broke up and ﬁgures staggered away. Only the beast lay still, a few yards from the sea. Even in the rain they could see how small a beast it was; and already its blood was staining the sand.
Simon is dead, killed by the other boys, and the next day only Ralph seems to feel any sense of responsibility or remorse for this act.
While Simon's death was an accident, Piggy's was not. When Jack comes and steals Piggy's glasses one night, Piggy has had enough. While he has always been afraid of Jack--and probably still is, to some extent--he is ready to fight back. It is not going to be a fair fight, however, because Piggy can barely see and, though he has Ralph and the conch, Jack has a tribe of savages who no longer recognize the authority of the conch.
Piggy asks “Which is better—to have rules and agree, or to hunt and kill?” It is not a difficult question for Jack, and he allows Roger to lever a boulder to drop on Piggy, smashing both the boy and the conch.
When the naval officer arrives to rescue the boys, Ralph
Ralph wept for the end of innocence, the darkness of man’s heart, and the fall through the air of the true, wise friend called Piggy.