assassin vs serial 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. 

serial killer

noun
  • A person who commits multiple (more than two) murders, especially similar ones with no obvious motive over a period of time with a "cooling-off" period between each murder. 

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