fratricide vs kingslayer

fratricide

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

  • A person who commits fratricide. 

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

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

kingslayer

noun
  • One who kills a king. 

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