Because they know what they are becoming and don't want to believe it. "you were outside. Outside the circle. You never really came in. Didn't you see what we - what THEY did?" This shows that Piggy and Ralph are in denial of their actions because they don't want to believe them theirselves.
Piggy, Samneric, and Ralph lie about Simon's death because they are afraid of taking responsibility for their part in the violent mob that killed him. They are overwhelmed by guilt and fear of being held accountable for their actions. Denying the truth allows them to distance themselves from the horrific event.
Sam and Eric justify their involvement by explaining that they were caught up in the frenzy of the moment and were too scared to stop the others from attacking Simon. Piggy justifies his involvement by claiming that he thought they were attacking the beast and not Simon. Ralph justifies his involvement by stating that he was a part of the group and felt pressured to go along with the others.
Ralph does not kill Piggy in William Golding's novel "Lord of the Flies." Piggy is killed when Roger, another boy on the island, pushes a rock onto him, causing him to fall to his death. Ralph is not directly involved in Piggy's death.
Simon's death causes an uproar from Ralph and Piggy who are furious about how and who murdered him. Samneric two of the people who murdered Simon, but due to shame, they decided not to tell Ralph or Piggy. Ralph secretly knew about it, but did not tell them.
The group of hunters and Ralph immediately reenact the killing of the pig by pretending to attack Robert as part of the hunt ritual. This disturbing act blurs the line between reality and savagery, foreshadowing the descent into violence and chaos on the island.
During the first meeting, which was held in chapter 1: The Sound of the Shell, Ralph was elected as the chief. Later in the book Jack formed his own tribe and appointed himself as chief.
Ralph is the only boy who truly recognises the significance of Simon's death. Piggy is ready to lay part of the blame on Simon himself and the dance, the Twins are in total self denial and Jack dismisses the entire episode as a 'trick' played by the beast. Ralph alone gives the deed its true name, murder, and Ralph alone seems to realises the depths to which they have sunk and the ferocity of their attack. Ralph too realises that having gone this far Jack and his tribe are now capable of almost anything.
Ralph is the only boy who truly recognises the significance of Simon's death. Piggy is ready to lay part of the blame on Simon himself and the dance, the Twins are in total self denial and Jack dismisses the entire episode as a 'trick' played by the beast. Ralph alone gives the deed its true name, murder, and Ralph alone seems to realises the depths to which they have sunk and the ferocity of their attack. Ralph too realises that having gone this far Jack and his tribe are now capable of almost anything.
There is never any mention of Simon's family. The only things known about Simon's life before his arrival on the island are what Jack reveals. Simon was part of the choir and was prone to fainting.Piggy told Ralph that both of his parents were dead and that he lived with his auntie, so Piggy was an orphan.
The rationalization is undertaken mainly by Piggy. Ralph is only too aware they the boys collectively murdered Simon is a frenzied attack and he is deeply upset and filled with remorse. Piggy tries to down play their role insisting that it was a combination of the storm, the dance and even Simon's own fault, for wandering around in the dark, which led to his death. Piggy's advice is to simply forget about it as it can't be changed and thinking about it serves no purpose.
Nothing
Ralph is as guilty as any of the other boys because he took part in the dance and the subsequent brutal killing of Simon. As he explained to Piggy in the following chapter, "That was murder." He also added "You never really came in. Didn't you see what we-- what they did?"