hatchet man vs man-killer

hatchet man

noun
  • A male professional killer or gunman. 

  • A male who carries out brutal and unpleasant duties on behalf of another, such as firing dead wood employees. 

  • A male hatchet-wielding participant in a Chinese-American tong war. 

man-killer

noun
  • A murderer. 

  • An animal that kills humans. 

  • Someone who habitually seduces men. 

How often have the words hatchet man and man-killer occurred in a corpus of books? (source: Google Ngram Viewer )