In Chapter 6 of Night, those who could not keep up with the forced march were killed by the SS. The officers were given orders to shoot at those who slowed down or showed signs of weakness. These orders were still in keeping with the Nazi agenda of the "Final Solution." The Germans did not care how many died because, at the core of the holocaust, death for all Jews was the final objective.
In Chapter 5, the Red Army was approaching Buna, and the SS had no choice but to abandon the camp together with the prisoners. At the time, Elie was recovering from a surgery on his foot, but he decided to go with his father on the march out of Buna. On the march, which begins in Chapter 6, the prisoners were forced to increase their pace, and some fell behind. Those who failed to keep up were shot.
The night was pitch-black. From time to time, a shot exploded in the darkness. They had orders to shoot anyone who could not sustain the pace. Their fingers on the triggers, they did not deprive themselves of the pleasure. If one of us stopped for a second, a quick shot eliminated the filthy dog.