This is a question that is hard to answer, because there are so many people in the book who did horrible deeds because of their prejudice. However, if I had to pick one, I would say that Dr. Mengele has to be one of the worst. He is mentioned several times in the book.
He was an officer in the SS (German Schutzstaffel) and he was also a physician in the notorious concentration camp of Auschwitz. He sent many Jews to death in gas chambers. He did all of these things with no compunction or sorrow. In fact, in Night, it is mentioned that he did his work with a smile, as if it were a game. He also performed experiments on prisoners. Here is a disturbing quote:
Yes, we were ready. So were the SS doctors. Dr. Mengele was holding a list: our numbers. He nodded to the Blockalteste: we can begin! As if this were a game.