In chapter 7“Beast from the Air,” the war gets closer. The parachutist is described as from “the world of grown-ups” (ch 7). The boys, however, exist in a fantasy world. In their minds, the parachutist becomes the beastie, the monster to be feared.
Earlier, the boys invented the story of the beast, or beastie. It was a natural response to fear of the island, being alone, and what they might become.
The beastie represents the boys’ savagery, and it certainly comes out when the parachutist drops down. The boys deal with their fear by attacking the unknown. This is how Piggy and Simon are murdered.
The twins, Sam and Eric, are watching the fire. The fall asleep, and when they wake up and build a new fire they see the parachutist. Terrified, they run back to wake the others, saying, “We saw the beast—” (ch 7).
An assembly is called, and Sam ’n Eric’s vivid imagination recalls a terrible beast, with fur, eyes, teeth, and claws. The boys decide to hunt it.
“This is more than a hunter’s job,” said Ralph at last, “because you can’t track the beast. And don’t you want to be rescued?” (ch 7)
The parachutist would make anyone frightened, dropping in unannounced. The boys’ response fits into their growing savagery at this point. By chapter 7, they are becoming less and less civilized. They forget the outside world so completely that in their minds, there is no parachutist—only a beast.
I believe you are talking about the boys from "The Lord of the Flies" right? From what I remember they thought the dead parachutist was the monster or something that the little boys were afraid of.