coup de grâce vs fratricide

coup de grâce

noun
  • A final blow or shot given to kill a wounded person or animal. 

  • A remarkable finishing action. 

fratricide

noun
  • The intentional or unintentional killing of a comrade in arms. 

  • A person who commits fratricide. 

  • The killing of one's brother (or sister). 

  • The undesirable situation where the separate missiles from a MIRV interfere with each other as they explode. 

How often have the words coup de grâce and fratricide occurred in a corpus of books? (source: Google Ngram Viewer )