Jack Ketch (1600-1686). 25-year English executioner He killed. He executed Charles I, Guy Fawkes, and Gunpowder Plotters.
Nikolay Yezhov (1895-1940) headed the NKVD during the Great Purge. He killed millions, including political prisoners, religious leaders, and civilians. He tortured, shot, and hung.
Imad ad-Din Zengi (1085-1146) 12th-century Aleppo and Mosul ruler. He fought cruelly. He skillfully executed several opponents.
Gilles de Rais (1404-1440), a French aristocrat, was accused of killing over 100 infants. He was judged guilty and burned at the stake. He was a nasty serial killer.
SS doctor Josef Mengele (1911–1979) was German. His Auschwitz concentration camp experiments were infamous. He killed over 400,000.