Impostor

A spaceship is travelling in endless space, and its crew is trying to survive. There is an impostor among them and they need to stay on guard. The Impostor wants to kill everyone and will stop at nothing, even if it means sabotaging the ship. He can hide in dark corners and wait for his victim, or suddenly jump out of a ventilation shaft and attack from behind. It is difficult to identify him among the crew members, unless he makes a mistake. You wouldn't do that, would you, Impostor?