Golding uses a variety of literary device and plot to convey the idea that this tropical island is indeed a dangerous place.
First, Golding's inciting incident for Piggy is the notion that there are no parents or adults to supervise the children:
“This is an island. At least I think it’s an island. That’s a reef out in the sea. Perhaps there aren’t any grownups anywhere.”
The thought grows into reality a few pages later.
"Didn’t you hear what the pilot said? About the atom bomb? They’re all dead."
Ralph had believed his dad would rescue them, but the reality of the absence of adults is starting to sink in for Ralph too. Children without the protection of adults is dangerous.
The boys discover that the island is indeed uninhabited, but for a few pigs:
They found a piglet caught in a curtain of creepers, throwing itself at the elastic traces in all the madness of extreme terror.
Golding uses fearful diction to allude to coming danger: ill-omened, whisper, creepers, crept, and quivered. Golding uses the setting (the air grows increasingly hot as the realization of their situation unfolds) to portray danger.