first-degree murder vs murderer

first-degree murder

noun
  • An act that constitutes the crime of first-degree murder. 

  • The criminal offence of murder in a form considered by law to be the most serious; the precise distinction may vary by jurisdiction but typically involves premeditation, or the choice of victim or other circumstances being regarded as especially blameworthy. 

murderer

noun
  • A person who commits murder. 

How often have the words first-degree murder and murderer occurred in a corpus of books? (source: Google Ngram Viewer )