In Curtain, the last Hercule Poirot novel, Edward Riggs is an agricultural labourer. Riggs suspected that his wife was having an affair with the lodger Ben Craig. Both Craig and Mrs Riggs were found murdered, shot with Riggs' gun. Edward declared that he must have killed them but he couldn't remember. He would have been sentenced to death but the final verdict was penal servitude for life.
In Poirot's manuscript it is revealed to Captain Hastings (and to the reader itself) that Edward Riggs had been mentally manipulated by Stephen Norton into killing his wife and her lover.