corpse than a living, breathing human. This is the direct result of all the many horrors he's had to endure at the hands of the Nazis. Many of his fellow Jews have been reduced to literal corpses, murdered in their millions by the Nazis; but Elie has become a figurative corpse in that, though he's still very much alive, he looks as if he's dead.

While I completely agree with the other answers to this question, I would also point out that Elie refers to himself as a corpse because that was how he literally appeared. He had spent a year in Nazi concentration camps and had been on a starvation diet the entire time. Take a look at the photographs or newsreel footage of the liberation of the camps and you'll see the walking corpses who were discovered by American and Russian troops in the aftermath of World War II. Wiesel recounts the harrowing forced march from Buna to Gleiwitz and then the train ride from Gleiwitz to Buchenwald. During this journey, the prisoners received almost no food. Wiesel points out the despicable behavior of the German civilians who threw food into the train cars just to see the starving men kill each other to get a morsel of bread. By the time he reached Buchenwald, Elie had obviously lost much of his body weight, his eyes had become sunken in his head and he very much resembled a corpse.

In Night, Elie Wiesel recounts his time in Auschwitz. The Jewish people suffered such horrors that most of them were killed or died while being held captive. Elie goes into great detail of the horrors suffered at the hands of the Nazis. Elie lost so much while he was a prisoner. His entire family was lost, he lost his faith, and he almost lost his life. Throughout the book Elie questions God about what is happening to him and the other Jews.

By the end of the book, Elie is barely alive. As he gazes into the mirror after being freed from the camps, Elie looks at his reflection. He questions whether he is still inside the man now gazing back at him. Elie wonders if he has seen too much and gone too far. He questions whether the old Elie will ever return. Elie considers himself a walking corpse at this point. 

Elie's struggles would have killed the strongest of people, but Elie survived. He wonders whether he will ever be able to live a normal life again. Elie Wiesel has gone on to become a renowned man working for peace. He didn't let the horrors of what happened to him destroy him, instead he uses what happened to him as a stepping stone to help others. 

The last lines of the Elie Wiesel's novel, Night, is as follows:

From the depths of the mirror, a corpse gazed back at me. The look in his eyes, as they stared into mine, has never left me.

Here, Eliezer Wiesel is ending his recollection of his time as a prisoner and survivor of the Holocaust. The last lines of the text leave the reader with a real and horrifying image of a survivor's inner self.

Wiesel is stating that while he was able to survive the concentration camps he was held prisoner in, he fails to see himself as truly alive. The experiences he faced, the horrors he witnessed, and the terrors he lived killed him on the inside. Even though he survived physically, he no longer recognizes himself. His soul and body have separated.

This idea is compounded by allotting a separate being to the image in the mirror. Wiesel is no longer whole. But, hope can be found (even as dark as the passage is). Wiesel recognizes that his "being" as part of the camps is dead. Wiesel is left with the fact that he did survive, while badly battered and broken.

I think that the most significant element of the last line brings to light how Eliezer is incapable of recognizing his own reflection.  On a physical level, it brings light to the significant manner in which Eliezer has changed over his experience of surviving the ordeal outlined.  On a more psychological level, the inability to recognize himself brings to light how much Eliezer has changed.  The ideas and hopes, promises and possibilities, in which Eliezer believed are now long gone.  At the start of the book, Eliezer was someone who believed in God, hoping to follow Moshe's advice of gaining the courage to ask God the questions that plague his being.  He was a boy who believed in his community and in his family, a boy who possessed a healthy respect for his father and the role he played in the community as a leader.  By the end of the narrative, Eliezer has effectively renounced his spiritual side, lost connection with all others, and sought only his own survival, akin to an animal.  He no longer viewed his father with the type of reverence that was seen at the start of the narrative, rebuking his father at different points, and not responding to his cries of suffering.  The last moments Eliezer has of his father is of his wailing in pain, and Eliezer almost secretly hoping that he would "go away."  When Eliezer cannot recognize his reflection, it is a significant instant where he has changed so much that he cannot fully embrace or appropriate who he now is from what he once was.