In chapter 6 of "Night," how did Zalman die?

Chapter six of Night finds the prisoners being forced on a march. There are having to march in a blizzard. They end up marching 42 miles. Zalman, a fellow prisoner, is running beside Elie. He complains of stomach cramps and wants to stop. Elie begs him to keep running, because if the SS were to see any of them not running, they would kill them. Zalman says he can't keep going and squats to the ground. Elie says that he never sees him again. Zalman was trampled by the other prisoners.

Elie has an injured foot, but keeps up the pace for his father. They are running together and keep each other going. Zalman was killed because the prisoners were afraid to stop. Elie says that if the SS saw any of them stop running they would kill them. The prisoners were so afraid of the SS that they trampled over one of their own, just to keep the German officers from killing them. 

This march shows us the strength that Elie has. He is injured, but pushed himself for the sake of this father. He sees Zalman go to the ground, and knowing he would never see him again, he still has the strength to keep going. The human will is a force all its own. Elie has the desire deep inside of him to survive, although all around him is nothing but death.