Kai Höss can’t remember if he was in sixth or seventh grade on the day he learned that his grandfather Rudolph Höss, a German SS officer during World War II, was likely the greatest mass murderer in human history. But the now 63-year-old evangelical pastor from Stuttgart, Germany, can still vividly recall the unshakeable sense of “shame” he felt after reading about the commandant of the Auschwitz concentration camp — where approximately 1 million people were killed — in a school textbook, then coming home and asking his mom why he and Rudolph shared the same last name. “That’s your...