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.