Golding uses a number of words and phrases in the first chapter of the novel that make the island seem a bit threatening. Explain why he used them.

Lord of the Flies is not a happy book. It's a dark and depressing account of young boys foregoing societal kindness and altruism in favor of base savagery. Golding begins setting this mood right from the start of chapter 1. He is foreshadowing to readers the foreboding effects that the island will have on the boys.
The island is a tropical island, and Golding absolutely could have initially painted it as a paradise, but he doesn't. Golding wants that scary mood and tension building right from the start. In the first paragraph, readers get words like "scar," "smashed," "heat," "creepers," and "witch-like." The setting already sounds like a horrible place to be. It's a place that has seen rough weather and is broken because of it.

He was clambering heavily among the creepers and broken trunks. . . .

A page or two later, readers get a sizable paragraph that describes the area around Ralph and Piggy. The broken and battered descriptions continue, and there are a couple of words used that convey overtones of death.

The ground beneath them was a bank covered with coarse grass, torn everywhere by the upheavals of fallen trees, scattered with decaying coconuts and palm saplings. . . .
Then he leapt back on the terrace, pulled off his shirt, and stood there among the skull-like coconuts with green shadows from the palms and the forest sliding over his skin.

Golding's word choice in the opening pages does a great job of alerting readers to the fact that the coming events are going to be dark and dangerous.