murderer

n. one who illegally kills another person

Murder is the unlawful killing of one human being by another. Murder is generally distinguished from other forms of homicide by the elements of malice aforethought and the lack of justification. All jurisdictions, ancient and modern, consider it a most serious crime. Most jurisdictions impose a severe penalty for its commission.
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Murder, n the intentional termination of another human being's life through physical, chemical, or neglectful means. [more]Murderer - Community and Resources

Noun
1. a criminal who commits homicide (who performs the unlawful premeditated killing of another human being)
(synonym) liquidator, manslayer
(hypernym) criminal, felon, crook, outlaw, malefactor
(hyponym) assassin, assassinator, bravo
(derivation) murder, slay, hit, dispatch, bump off, polish off, remove

(n.)
One guilty of murder; a person who, in possession of his reason, unlawfully kills a human being with premeditated malice.
  

(n.)
A small cannon, formerly used for clearing a ship's decks of boarders; -- called also murdering piece.