The Collaborative International Dictionary of English v.0.48:

Murderer \Mur"der*er\ (m[^u]r"d[~e]r*[~e]r), n.
   1. One guilty of murder; a person who, in possession of his
      reason, unlawfully kills a human being with premeditated
      malice.
      [1913 Webster]

   2. A small cannon, formerly used for clearing a ship's decks
      of boarders; -- called also murdering piece. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

murderer
    n 1: a criminal who commits homicide (who performs the unlawful
         premeditated killing of another human being) [syn:
         murderer, liquidator, manslayer]

Moby Thesaurus II by Grady Ward, 1.0:

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