Why does Elie say, "there is no more reason to live, no more reason to struggle" when he wakes up to a snow covered, open cattle car?
Elie had believed his father to be dead, and had lost all reason/will to keep fighting.
Why does Elie, some years later, ask a lady not to throw money out to the poor?
Because the kids were hurting each other for the money, and it reminded him of his fellow prisoners doing so for food.
How is the passage describing the fight over a piece of bread in the open cattle car one of both horror and sadness?
People were killing each other, an act no ordinary person should be able to commit casually, for a mere piece of bread.