What happened in the concentration camps that shows that Eliezer is becoming "hardened" as a result of being a prisoner?
Simply put, Eliezer was forced to witness some of the most horrific conditions that one can imagine. Consider his writing at the first sites of the death camp. The separation from his mother, the burning of bodies, the murder of children are all events that compel Eliezer to "never forget." In these opening instances, there is much in way of a sense of hardening that is happening to him. Over the course of his narrative, Eliezer experiences more instances of despair and agony than redemption. Along with these, there is greater exposure to abuse and wanton cruelty that Eliezer sees to make him even more despairing of human beings. For example, when Eliezer and the other prisoners see the little boy being hung, it is a moment where a statement about the lack of hope or redemption is being clearly made. When the little boy takes longer to die because his head does not exactly fit the contours of the noose, the prisoners are forced to watch him convulse and shake for about thirty minutes. Seeing this moment, Eliezer hears the others say that God is "there" in that "He is hanging on this gallows." Accordingly, Eliezer begins to take a hardened stance towards the power of the divine, in suggesting that a merciful or compassionate God could not have conceived of what those in the death camps had to endure. This is reflected in Eliezer's assertion that Hitler "was the only one to keep his promises." Such a statement reflects the condition of despair and emotional hardening that has taken place in Eliezer as a result of his exposure to the worst of human cruelty.
Wiesel begins to "harden" almost immediaely upon his arrival at Buna. When his own father is stuck by a guard, Elie is angered but does nothing to help his father. Later when Wiesel's father happens to get in the way of Idek, the mad Kapo, and get beaten Elie only feels anger. However, the anger is not directed at Idek but at his own father for not being able to stay out of Idek's path of madness and rage. Wiesel knows his thinking is not rational as he has been punished by Idek during a bout of his madness but Wiesel cannot help the anger he feels towards his father.