There are several themes present throughout of the book. One of the most important is the idea that the actions of man can dehumanize other men. The writer has experiences that "dehumanize" him, meaning that he loses the qualities (to some extent) that make him what is, the defining aspects of his humanity.
We see this in his response to his father. Near the end, Elie struggles emotionally as his father's health declines. Prior to this, Wiesel told the reader a story about the Rabbi Eliahou and his son in which the son leaves the father behind on the forced march, presumably because he believes the father is moving too slowly. The son wants to save himself, so he lets his father fall behind. When Elie hears Rabbi Eliahou tell this story he prays that God will never allow him to leave his own father behind.
However, by the end of the book, Elie has thoughts of doing just that. As his father is dying of dysentery, Elie thinks to himself that he would be better off if his father would just go ahead and die. Once he does die, he briefly thinks "free at last!" Elie suffers with guilt over these thoughts. But his experience with suffering, cruelty, and death have changed him and he can no longer look at things the way he did before his imprisonment.
See the link below for other themes in the book.