why is hanging of the "sad eyed angel" one of the most profoundly moving events in the novel?

There are few parts of books I refuse to read again. One is the "sad eyed angel" from Night. Eliezer's description of the child hanging casts a shadow over any redemption for humanity. The hanging of a child reminds me that evil exists and is personified in atrocities like the Holocaust. How can this be? Beyond the psychological explanations of brainwashing and mad fascism, I can only surmise that the humanity is somehow broken.