The Collaborative International Dictionary of English v.0.48:

hit man \hit man\ n.
   1. A professional murderer, esp. one working for a criminal
      organization; also called torpedo. [Colloq.]
      [PJC]

   2. A slanderer working for political purposes to damage the
      reputation of an opponent; a hatchet man.
      [PJC]

WordNet (r) 3.0 (2006):

hit man
    n 1: a professional killer who uses a gun [syn: gunman,
         gunslinger, hired gun, gun, gun for hire,
         triggerman, hit man, hitman, torpedo, shooter]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "hit man":
   Cain, apache, assassin, assassinator, bloodletter, bloodshedder,
   bravo, burker, butcher, button man, cannibal, cutthroat, desperado,
   eradicator, executioner, exterminator, garroter, gorilla, gun,
   gunman, gunsel, gunslinger, hatchet man, head-hunter,
   homicidal maniac, homicide, killer, man-eater, man-killer,
   manslayer, massacrer, matador, murderer, pesticide, poison,
   poisoner, slaughterer, slayer, strangler, thug, torpedo,
   trigger man