assassination vs serial killer

assassination

noun
  • The murder of a person, especially for political reasons or for personal gain. 

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 assassination and serial killer occurred in a corpus of books? (source: Google Ngram Viewer )