noun (n)

• someone who causes the death of a person or animal(noun.person)

  Synonym:
  slayer
  

  source: wordnet30
• the causal agent resulting in death(noun.state)

  Example:
  Heart disease is the biggest killer in the United States.
  

  Synonym:
  cause of death
  

  source: wordnet30
• a difficulty that is hard to deal with(noun.cognition)

  Example:
  That exam was a real killer.
  

  source: wordnet30
• predatory black-and-white toothed whale with large dorsal fin; common in cold seas(noun.animal)

  Synonym:
  grampus, killer whale, orca, orcinus orca, sea wolf
  

  source: wordnet30
• One who deprives of life; one who, or that which, kills.(noun)

  source: webster1913