killer

noun

1. someone who causes the death of a person or animal

   noun.person

   Synonyms slayer

   Source: WordNet 3.0

2. the causal agent resulting in death

   noun.state

   Examples

   Heart disease is the biggest killer in the United States.

   Synonyms cause of death

   Source: WordNet 3.0

3. a difficulty that is hard to deal with

   noun.cognition

   Examples

   That exam was a real killer.

   Source: WordNet 3.0

4. predatory black-and-white toothed whale with large dorsal fin; common in cold seas

   noun.animal

   Synonyms grampus killer whale orca orcinus orca sea wolf

   Source: WordNet 3.0

5. One who deprives of life; one who, or that which, kills.

   noun

   Source: webster1913