Why does he call himself a corpse, and what does this signify about the way his life was changed forever?

There are layers to this question. Elie's faith has died. He no longer believes in his God. Elie's sense of community is dead too. He has seen things that Jews have done to each other out of desperation. He no longer feels the community he once felt in Sighet. Finally, the death of his father killed something in Elie. I don't think anyone could be the same after surviving a Nazi concentration camp.