The boys kills Simon in a frenzy. They would like to believe it was self defense, but that isn't totally true. Ralph is the only one to acknowledge this:
“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.”
Piggy stops Ralph from calling it murder, and explains that it happened because they were scared. Ralph acknowledges that he wasn't really scared, and was really just caught up in the frenzy and wildness of the dance. Piggy convinces Ralph it was an accident.
“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."
When Sam and Eric join Piggy and Ralph, all four of them have moved on to pretending none of them were there. They all claim they left early.
Meanwhile, Jack and the rest of the tribe cope with the tragedy by refusing to acknowledge that it was Simon they attacked.
“—and then, the beast might try to come in. You remember how he crawled—”
The semicircle shuddered and muttered in agreement.
“He came—disguised. He may come again even though we gave him the head of our kill to eat. So watch; and be careful.”
They explain it was the beast in a disguise. Furthermore, they convince themselves that they didn't even kill it.
“But didn’t we, didn’t we—?”
He squirmed and looked down.
In the silence that followed, each savage flinched away from his individual memory.
“No! How could we—kill—it?”
Half-relieved, half-daunted by the implication of further terrors, the savages murmured again.
If they pretend Simon was really the beast in disguise, that excuses their attack. And if it was this powerful beast, then their attack couldn't have actually killed it. This explanation is how the boys cope with their actions, instead of facing the reality of murder.