hecatomb vs mass murder

hecatomb

noun
  • Any great sacrifice; a great number of people, animals or things, especially as sacrificed or destroyed; a large amount. 

  • A great feast and public sacrifice to the gods, originally of a hundred oxen. 

mass murder

noun
  • The killing of a large number of people over a short period of time. 

How often have the words hecatomb and mass murder occurred in a corpus of books? (source: Google Ngram Viewer )