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Someone was throwing stones: Roger was dropping them, his one hand still on the lever. Below him, Ralph was a shock of hair and Piggy a bag of fat. (11.198)
From Roger's vantage point (literally and figuratively), Ralph and Piggy have no humanity. He reduces them to "hair" and "fat," because if they have no identities then he's not really a cold-blooded murderer.
Quote Ralph listened […]. He had even glimpsed one of them, striped brown, black, and red, and had judged that it was Bill. But really, thought Ralph, this was not Bill. This was a savage whose image refused to blend with that ancient picture of a boy in shorts and shirt. (12.2)
Ralph is doing what we aren't allowed to do: he's convinced himself that these little terrors aren't the same boys he knew before. If they were, he'd have to face the fact that they all—including himself—are evil and savage. It's a lot easier for him to say "this [is] not Bill." Is he right? Or are the "savage" and "Bill" one and the same?
The boys, their bodies streaked with colored clay, sharp sticks in their hands, were standing on the beach making no noise at all. (12.219)
When the naval officer shows up, the "painted savages" turn back into "little boys" covered with clay and holding sharp sticks. Ralph is no longer afraid of them in the presence of this representative of civilization and the adult world—even though the boys could conceivably overpower the adult and send him the way of Simon and Piggy.