On June 17, 1794, Charles-Henri Sanson, high executioner of the First French Republic, guillotined more than 50 “conspirators” in 28 minutes, including an 18-year-old girl who seemed so fragile that “a tiger would have pitied her.” That night he wrote in his diary: Terrible day. The guillotine devoured 54. My strength went, my heart failed me. That evening, sitting down to dinner, I told my wife that I could see spots of blood on my napkin. … I don’t lay claim to any sensibility I don’t possess: I have seen too often and too close up the sufferings and death...