[1] : 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
see also:
orca killer whale orca gladiator orca ater orca rectipinna 

[2] : WordNet (r) 2.0

killer
     n 1: someone who causes the death of a person or animal syn: slayer
     2: the causal agent resulting in death; "heart disease is the
        biggest killer in the united states" syn: cause of death
     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, orca, grampus,
         sea wolf, orcinus orca
see also:
slayer cause of death killer whale orca grampus sea wolf 
orcinus orca 

[3] : 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

Killer >> K Words
Killer, definition of term: Killer
killer_pag1.html

Killer Whale >> K Words
Killer Whale, definition of term: Killer Whale
killer+whale_pag1.html

Killer Bee >> K Words
Killer Bee, definition of term: Killer Bee
killer+bee_pag1.html

Killer App >> K Words
Killer App, definition of term: Killer App
killer+app_pag1.html

Serial Killer >> S Words
Serial Killer, definition of term: Serial Killer
serial+killer_pag1.html

Lady Killer >> L Words
Lady Killer, definition of term: Lady Killer
lady+killer_pag1.html

Cicada Killer >> C Words
Cicada Killer, definition of term: Cicada Killer
cicada+killer_pag1.html