In chapters 1-5, Elie Weisel and the Jews he is with are still hopeful and full of faith. They continue to believe that nothing horrible will happen, that they will be saved, that life will return to normal. In these chapters, everyone is still human. They are still full of human kindness and caring for those who are suffering. They have not yet sunk to the level of animals, even though they are treated in this manner.
By chapters 6-9, reality has set in and the language is darker, more depressing and lacking that hope of salvation. Elie has lost his faith in God, as many of the Jews who have witnessed such atrocities against humanity (babies, children, the elderly) have. At this point in the book, it is everyone for himself. All bonds between friends and family members no longer exist. Survival is everything, and one may sacrifice his own father for a small piece of bread to calm his aching stomach. They have succumbed to the de-humanization of the Nazis in these atrocious camps.