Why does Golding decide to introduce the parachutist in chapter 6?

The entrance of the parachutist in chapter six is notable for both its timing and its symbolism. In chapter five, Ralph begins to understand his limits as a leader. He has the ability to get the other boys to listen to him, but he relies on Piggy's intelligence to make good choices and set policies on the island. Jack becomes more of an impediment, because he does not understand the fear that some of the boys have. His lack of regard for the dangers of psychological harm is becoming a threat because of the sadistic behavior he, Roger, and Maurice exhibit. Chapter five ends with Piggy worrying aloud that Jack will turn to physical violence against himself and the others.
Chapter six opens with imagery of a faraway dogfight, which provides the circumstances that make the appearance of the dead parachutist plausible. It also reinforces the mood of foreboding already established in chapter five. It is another reminder of the essential conflict of man versus man, playing out in both the adult world of military might between warring nations and the escalating tensions among the boys on the island. When the parachutist is observed by the boys, their perception is that the corpse is alive, as it moves up and down, entangled in the parachute cords and foliage. They don't understand what they are seeing, and fear overtakes rationality. This "beast from the air" rattles the boys so much that the conch is dispensed with, and the tenuous democracy of the island becomes doomed.

In chapter 5, Ralph holds an assembly to discuss why the boys are neglecting their responsibilities and address their fear of the beast. Essentially, the boys' attempt at creating a civil society is failing, and they are gradually descending into savagery. Towards the end of the meeting, Jack openly defies Ralph and leaves the assembly unexcused. After the hunters follow Jack, Ralph and Piggy lament about their terrible situation and ask for a sign from the adult world.
At the beginning of chapter 6, a paratrooper is shot out of the sky and lands on the top of the mountain. The dead paratrooper's arrival is both ironic and significant. The dead paratrooper's ominous arrival symbolizes the presence of evil on the island and indicates that the boys' situation will only get worse. Golding strategically chose to introduce the dead paratrooper in chapter 6 because it is a turning point in the novel when the boys' circumstances become dramatically worse. Once the paratrooper lands on the island, the boys becoming increasingly savage and end up brutally murdering Simon three chapters later.