What is the meaning of killer?

noun

• Someone who causes the death of a person or animal.
  • synonyms: slayer
• The causal agent resulting in death.
  • usage: "heart disease is the biggest killer in the United States"
  • synonyms: cause of death
• A difficulty that is hard to deal with.
  • usage: "that exam was a real killer"
• Predatory black-and-white toothed whale with large dorsal fin; common in cold seas.
  • synonyms: killer whale, orca, grampus, sea wolf, Orcinus orca