In chapter twelve of Lord of the Flies, by William Golding, Ralph is being pursued by Jack and his hunters. Piggy is dead, deliberately killed when Roger and the others levered a boulder onto him and crushed him and the conch, the last symbol of civilization on the island. Things now are worse than they have ever been, and Ralph is fleeing in order to save his life.
Despite the seriousness of his situation and his recognition that the command to kill him is real, Ralph does not want to believe that the boys on this island, including himself in the case of Simon, would intentionally murder anyone.
The breaking of the conch and the deaths of Piggy and Simon lay over the island like a vapor. These painted savages would go further and further. Then there was that indeﬁnable connection between himself and Jack; who therefore would never let him alone; never.
He paused, sun-ﬂecked, holding up a bough, prepared to duck under it. A spasm of terror set him shaking and he cried aloud.
“No. They’re not as bad as that. It was an accident.”
Admitting that the most recent death, Piggy's, was a deliberate act of murder increases Ralph's fear for his own life. He knows there can be no reasoning or discussion which will save him; the savage who finds him will undoubtedly kill him. Even when the truth is obvious, Ralph tries to avoid believing it because he knows what it will mean for his own fate.
If the fire designed to smoke Ralph out of hiding (which it does) does not also bring a naval vessel to the rescue, Ralph would certainly have been the next murder victim on this island.
Ralph isn't able to admit Piggy's death was an accident because he was there when Piggy got killed.