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]