what does jack say to explain simons murder? how does this fullfill the prophecy of the LORD OF THE FLIES?

As I recall, a littlun thinks they killed the Beast and Jack says they didn't and it came back. I don't recall Jack ever making a formal statement about Simon's death. I know Piggy complained that he had no business sneaking out at night like that. The LOF turns out to be prophetic. The boys do kill Simon as he tries to deliver the good news. In true Christ-figure fashion, he is killed trying to save the boys from themselves.

Simon doesn't speak in Chapter 9 except to urge the boys in their killing of the beast. When he next appears again in Chapter 10, he tells the boys that they must be on watch for the others.......... that the beast may try to sneak in again, and he reminds them how the beast (Simon) crawled.

“Tomorrow,” went on the chief, “we shall hunt again.”

He pointed at this savage and that with his spear.

“Some of you will stay here to improve the cave and defend the gate. I

shall take a few hunters with me and bring back meat. The defenders of

the gate will see that the others don’t sneak in.”

A savage raised his hand and the chief turned a bleak, painted face

toward him.

“Why should they try to sneak in, Chief?”

The chief was vague but earnest.

“They will. They’ll try to spoil things we do. So the watchers at the

gate must be careful. And then—”

The chief paused. They saw a triangle of startling pink dart out, pass

along his lips and vanish again.

“—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.”