killer

(noun) someone who causes the death of a person or animal. Synonyms: slayer.

(noun) the causal agent resulting in death. Synonyms: cause of death. Example: "Heart disease is the biggest killer in the united states."

(noun) a difficulty that is hard to deal with. Example: "That exam was a real killer."

(noun) predatory black-and-white toothed whale with large dorsal fin; common in cold seas. Synonyms: killer whale, orca, grampus, sea wolf, Orcinus orca.