The Collaborative International Dictionary of English v.0.48:

hatchet man \hatchet man\ n.
   1. A person hired to murder or physically attack another; a
      hit man.
      [PJC]

   2. A person who deliberately tries to ruin the reputation of
      another, often unscrupulously, by slander or other
      malicious communication, often with a political motive,
      and sometimes for pay.
      [PJC] Hatchettine

WordNet (r) 3.0 (2006):

hatchet man
    n 1: a professional killer [syn: hatchet man, iceman]
    2: one whose job it is to execute unpleasant tasks for a
       superior [syn: hatchet man, enforcer]

Moby Thesaurus II by Grady Ward, 1.0:

130 Moby Thesaurus words for "hatchet man":
   Cain, Thersites, apache, assassin, assassinator, backbiter,
   battler, belittler, belligerent, belted knight, bickerer, blade,
   bloodletter, bloodshedder, bravo, brawler, bruiser, bully,
   bullyboy, burker, butcher, button man, calumniator, cannibal,
   caustic critic, combatant, competitor, contender, contestant,
   cutthroat, cynic, debunker, decrier, defamer, deflater,
   depreciator, derogator, desperado, detractor, disparager,
   disputant, duelist, enforcer, eradicator, executioner,
   exterminator, fencer, feuder, fighter, fighting cock, foilsman,
   gamecock, garroter, gladiator, goon, gorilla, gun, gunman, gunsel,
   gunslinger, hanger-on, head-hunter, heeler, hellion, henchman,
   hit man, holy terror, homicidal maniac, homicide, hood, hoodlum,
   hooligan, jouster, killer, knight, knocker, libeler, man-eater,
   man-killer, manslayer, massacrer, matador, militant, muckraker,
   mudslinger, mug, mugger, murderer, muscle man, pesticide,
   plug-ugly, poison, poisoner, quarreler, railer, rioter, rival,
   rodman, rough, roughneck, rowdy, ruffian, sabreur, scrapper,
   scuffler, slanderer, slaughterer, slayer, social critic, squabbler,
   strangler, strong arm, strong-arm man, strong-armer, struggler,
   swashbuckler, sword, swordplayer, swordsman, terror, thug, tilter,
   torpedo, tough, traducer, trigger man, tussler, ugly customer,
   ward heeler, wrangler