The theme in Night could probably best be stated as follows: Human beings have the ability, and sometimes the desire, to hatefully destroy the lives of others.
We see this happen in Night as the plot unfolds. In the beginning of the book, the part of plot that we would call the basic situation or exposition, the author sets up the story by showing how important religion is to the main character, himself, as a boy.
As the plot develops, the main character is subjected to cruelties and indignities that cause him to doubt his faith. In fact, he appears to completely reject his faith over the course of the book. Elie Wiesel, the author, structures the plot so that we see the effect that each new horrifying event has on the psyche of the young protagonist.
Finally, at the end of the book, the plot’s resolution, Wiesel looks at himself in a mirror. He writes that what he sees looking back at him is the face of a corpse. In other words, the plot has taken him from being an idealistic, religious teenager to a person essentially devoid of the qualities that make someone a living human being.