Simon's death happened as a result of crazed bloodlust, mistaken identity, and poor timing, not premeditated murder.
Samneric, Piggy, and Ralph originally joined the hunters' circle due to the approaching thunder storm:
"Piggy and Ralph, under the threat of the sky, found themselves eager to take a place in this demented but partly secure society" (152).
When Simon comes out of the forest to join the circle, the boys have been playing a hunting game, where Roger pretended to be a pig, then a hunter. Simon is not perceived as a being one of the boys, but rather "a thing" that crawled "darkly, uncertainly" (152).
Later that night, Ralph tells Piggy "That was Simon [...] That was murder" (156).
Ralph participated in the boys' attack and killing of Simon. He tells Piggy who wondered if maybe Simon was pretending:
"You were outside. Outside the circle. You never really came in. Didn't you see what we--what they did?" (157)
Ralph includes himself in the mob with the collective use of "we," and then corrects himself to "they" in an attempt to separate himself from the violence. Ultimately, Ralph was on the inside of the circle that attacked Simon. He may not have had participated as actively as some of the other boys like Jack and Roger, but Ralph did not also try to stop the violence, either.
When Samneric join Piggy and Ralph, they also avoid connecting themselves to what happened with Simon, but their reluctance and reactions reveal that they too were there. The twins claimed that they left the firewood and were gathering firewood:
"'Yes. We were very tired,' repeated Sam, 'so we left early. Was it a good--'
The air was heavy with unspoken knowledge. Sam twisted and the obscene word shot out of him. '--dance?'" (158)
All of the boys share in the culpability of Simon's death.