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]