A meteor shower damaged the air supply of the spaceship. Originally there was enough air for the pilot and five passengers. The damage left enough for only five people. The pilot offered to sacrifice himself but the passengers doubted they could land the ship without him. He was asked to choose one of them for death.
When the pilot claimed that the air supply had dwindled to the point of only being enough for one, Darius jumped at the opportunity. He drew a gun and threaned the others at gunpoint. He wanted to enter the supposedly safe air storage compartment alone. He did but it was no storage compartment. It was an escape hatch which launched him to outer space, where he was to suffocate within seconds.
The pilot explained that his original assesment of having enough air for five was true. He wanted the most selfish man onboard to reveal himself. He had manipulated Darius into killing himself, allowing everyone else to survive.