The Collaborative International Dictionary of English v.0.48:

Killer \Kill"er\, n.
   1. One who deprives of life; one who, or that which, kills.
      [1913 Webster]

   2. (Zool.) A voracious, toothed whale of the genus Orca, of
      which several species are known; called also killer
      whale.
      [1913 Webster]

   Note: The killers have a high dorsal fin, and powerful jaws
         armed with large, sharp teeth. They capture, and
         swallow entire, large numbers of seals, porpoises, and
         dolphins, and are celebrated for their savage, combined
         attacks upon the right whales, which they are said to
         mutilate and kill. The common Atlantic species (Orca
         gladiator), is found both on the European and the
         American coast. Two species (Orca ater and Orca
         rectipinna) occur on the Pacific coast.
         [1913 Webster]

WordNet (r) 3.0 (2006):

killer
    n 1: someone who causes the death of a person or animal [syn:
         killer, slayer]
    2: the causal agent resulting in death; "heart disease is the
       biggest killer in the United States" [syn: cause of death,
       killer]
    3: a difficulty that is hard to deal with; "that exam was a real
       killer"
    4: predatory black-and-white toothed whale with large dorsal
       fin; common in cold seas [syn: killer whale, killer,
       orca, grampus, sea wolf, Orcinus orca]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "killer":
   Bluebeard, Cain, Mafioso, Young Turk, apache, assassin,
   assassinator, beast, beldam, berserk, berserker, bloodletter,
   bloodshedder, bomber, bravo, brute, bucko, bully, bullyboy, burker,
   butcher, button man, cannibal, cutthroat, daisy, demon, desperado,
   devil, dilly, dragon, eradicator, executioner, exterminator, fiend,
   fire-eater, firebrand, fury, garroter, goon, gorilla, gun, gunman,
   gunsel, hardnose, hatchet man, head-hunter, hell-raiser, hellcat,
   hellhound, hellion, hit man, holy terror, homicidal maniac,
   homicide, hood, hoodlum, hooligan, hothead, hotspur, humdinger,
   incendiary, lulu, mad dog, madcap, man-eater, man-killer,
   manslayer, massacrer, matador, monster, mugger, murderer,
   pesticide, poison, poisoner, rapist, revolutionary, rough, rowdy,
   ruffian, savage, she-wolf, slaughterer, slayer, spitfire,
   strangler, termagant, terror, terrorist, thug, tiger, tigress,
   torpedo, tough, tough guy, trigger man, ugly customer, violent,
   virago, vixen, wild beast, witch, wolf