From the days of the sword through the advent of the guillotine, Charles-Henri Sanson killed some 3,000 people during his bloody career. On Jan. 5, 1757, King Louis XV of France departed the Palace of Versailles. While he was walking toward his carriage, a strange man suddenly shoved past the palace guards, striking the king in the chest with a penknife. The assailant was arrested and the king was ushered inside, bleeding from what turned out to be a minor chest wound. No longer fearing for his life, King Louis’ concern shifted from his own bodily injury to the kind...