Definitions of killer:

• noun:   a difficulty that is hard to deal with

  Example: "That exam was a real killer"

• noun:   someone who causes the death of a person or animal
• noun:   predatory black-and-white toothed whale with large dorsal fin; common in cold seas
• noun:   the causal agent resulting in death

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

• name:  A surname (very rare: popularity rank in the U.S.: #62862)