assassin vs contract killer

assassin

noun
  • Someone who intentionally kills a person, especially a professional who kills a public or political figure. 

  • Any ruthless killer. 

  • A member of the Nizari Ismaili Muslim community of the Alamut Period. 

verb
  • To assassinate. 

contract killer

noun
  • A person who contracts to kill a specified person for an agreed sum of money. 

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