User blog comment:NerdWithAKeyboard/Pure Evil Villains Who Never Killed Anyone/@comment-5032898-20171219144129

(Spoiler Warning) Mr. X from Curtain: Poirot's Last Case counts as he never directly cross the line himself. Instead, he induced his victims with psychological torture and turned them into murderers who did his dirty work while X himself attempted to escape justice and ruin Poirot's reputation. In the end, Poirot had to kill X in order to end the madness and finished his final case successfully before his death of sickness. Because of his methods, Mr. X is definitely the most formidable murderer in a novel by Agatha Christie.