he cantor is a vivid presence in any Jewish congregation, responsible for song, often a man with an outgoing personality and a sense of social engagement. Such a cantor was Eleazar Bernstein, who lived with his wife Martha and their three children in the southwestern German city of Zweibrücken in the 1930s. Among other good deeds, Bernstein would visit Jews in the local prison to lift their spirits. There he befriended a guard, a police captain named Kurt Trimborn, with whom he would play chess.

On the night of Nov. 9, 1938, Germans destroyed hundreds of synagogues, including Bernstein’s. On the day after this national pogrom, the infamous Kristallnacht,Bernstein was arrested, along with thousands of other Jewish men throughout Germany, all bound for concentration camps. His neighbors looted his apartment, broke his windows, and stole his furniture. Bernstein’s two sons were too small to understand. Coming home to find a wreck, they amused themselves by throwing things through the gaping window frames. Martha made her way across the rioting city to find her husband’s police captain friend and ask for help. Trimborn told Martha to pack, released Eleazar, and escorted the family across the French border. The car was so full of suitcases that the children had to lie flat on top of them in the back seat.

Four decades later, from America, Bernstein sent Trimborn a letter. The two little boys had grown up to become engineers. His daughter was a teacher. There were grandchildren. All of this thanks to Trimborn.

The letter was written after Trimborn’s conviction for mass murder.

Not long after helping the Bernstein family, Trimborn joined the German security police. He was trained for a special task force, an Einsatzgruppe, which was sent behind the invading German army to the Soviet Union. When Trimborn joined his Einsatzgruppe in occupied Soviet Ukraine in October 1941, its men were already murdering entire communities of Jews. That December, as the Red Army halted the German advance and the Americans joined the war after Pearl Harbor, Hitler proclaimed that the Jews were responsible for Germany’s predicament. In 1942, Trimborn personally ordered that hundreds of Jews be murdered, and carried out neck shots himself. One day he herded 214 children from an orphanage into a gas van—a truck refitted so that its exhaust fumes were pumped into the hold rather than into the atmosphere. The children screamed and pounded the walls as they were asphyxiated. ...