lethal injection vs rub out

lethal injection

noun
  • A means of execution or of euthanasia in which a person is injected with a fatal dose of drugs. 

rub out

verb
  • To delete or erase or remove (something) by rubbing. 

  • To kill, especially to murder. 

How often have the words lethal injection and rub out occurred in a corpus of books? (source: Google Ngram Viewer )