In William Golding's Lord of the Flies, Simon is the character who is closest to understanding that the "beast" the rest of the boys fear is actually them. After he makes the discovery, he tries to tell the others. Unfortunately, his timing is awful, and he crawls (because he is weak at the time) into a chanting circle of all the other boys who are reenacting their latest successful pig killing. It is dark and stormy on top of the mountain, and the boys are all (even Ralph and Piggy) caught up in the frenzy and kill Simon as if he were a pig they had surrounded at a hunt.

Your question implies that none of the boys are aware of the significance of having helped murder Simon, one of their own; in fact, it seems clear that at least some of them are quite aware of what they did and what it means. Ralph, in particular, knows.

The morning after the incident, Ralph and Piggy wake up on the beach and do not even mention what happened to Simon for quite a long time. Finally Ralph broaches the subject but Piggy does not want to discuss it. Finally Ralph speaks the word he has been thinking: 

“That was murder.”
“You stop it!” said Piggy, shrilly. “What good’re you doing talking like that?”
He jumped to his feet and stood over Ralph.
“It was dark. There was that—that bloody dance. There was lightning and thunder and rain. We was scared!”
“I wasn’t scared,” said Ralph slowly, “I was—I don’t know what I was.”
“We was scared!” said Piggy excitedly. “Anything might have happened. It wasn’t—what you said.”

Samneric are also obviously ashamed (because they are aware) of what happened last night, but they insist that they were not even there--despite the fact that all of them know this is a lie.

Piggy is the intellectual, and Golding says he was "searching for a formula" to excuse or justify what happened to Simon. Nothing Piggy tries works with Ralph; he is perfectly aware that he and Piggy helped murder their friend.

“It was an accident,” said Piggy suddenly, “that’s what it was. An accident.” His voice shrilled again. “Coming in the dark—he hadn’t no business crawling like that out of the dark. He was batty. He asked for it.” He gesticulated widely again. “It was an accident.”

Ralph is still unmoved but says that he is now "frightened. Of us." He wants to go home, afraid of what they are all becoming. Perhaps the other boys are in denial or do not even realize exactly what happened to Simon, but Ralph knows and he understands that Simon's death means others may be killed--and of course he knows that he and Piggy will be the next targets.