We know that Elie Wiesel regained his faith in God and humanity. However, this does not come across very clearly in the book, Night. He has suffered tremendously and in this suffering, he has been transformed into a different man, a man that he did not want to be, but his circumstances "forced" him. At the end of the book, he looks into the mirror and he says: "From the depths of the mirror a corpse gazed back at me."
With this stated, there were many regrets. The greatest regret perhaps though is his relationship with his father. Eliezer is merely a youth in the book and he seeks to take care of his father. You can say that roles are reversed. As his father gets sicker, he despises helping him. This feeling is compounded, because in a concentration camp, as he says, "there are no father, brothers, or friends." He even says to himself, "If only I could get rid of this dead weight," speaking of his father. Immediately he confesses that he felt ashamed, or we can say regret.
Finally, when his father dies (1945, January 29). He feel more regret, as he feels relieved and does not cry.