(n.) One who inflicts punishment.

Cain Jack Ketch apache assassin assassinator beheader bloodletter bloodshedder bravo burker butcher button man cannibal chastizer cutthroat deathsman desperado discipliner electrocutioner eradicator executioner exterminator garroter gorilla gun gunman gunsel hangman hatchet man head-hunter headsman hit man homicidal maniac homicide killer man-eater man-killer manslayer massacrer matador murderer pesticide poison poisoner punisher slaughterer slayer thug torpedo trigger man