The manipulator as a villain is one of the more interesting types of criminal, because the nature of a manipulator's evil is revealed through the actions of others. The manipulator is a puppet master, pulling strings that make his hapless victims dance. He or she would never soil a hand by actually committing a crime, but beware that you are not persuaded to do his dirty work. Horror uses manipulators very well. Stephen King's Needful Things is a prime example. Mystery is not without its manipulators, though.
When I consider manipulators, none impresses me more than X in Agatha Christie's Curtain. In many ways Curtain is a very sad book. Poirot and Hastings have returned to Styles. Styles is not the stately country estate it once was. Poirot is very old. Considering that he was a retired Belgian detective at the beginning of the series, 'ancient' would probably be a more accurate description. He is confined to a wheelchair and suffering with a failing heart. Hastings is mourning the death of his wife and worried over the romantic entanglements of his daughter. Into this, Christie introduces X.
Poirot knows the identity of X and tells Hastings that a murder is about to occur, but not the identity of the victim. He gives Hastings newspaper clippings of five murder cases, each with a different person convicted for the crime. X is connected to every case, but is not the actual killer. Poirot steadfastly refuses to confide the identity of X to Hastings, because Hastings' face would reveal the truth.
The book makes us question whether X is the master puppeteer Poirot believes, or if, at last, the great detective has lost the deductive powers of his "little gray cells." Christie's final twist is a surprising bit of manipulation that saddens us as it restores our faith in her storytelling ability. Few writers today have the skill to craft such an excellent example of the inner workings of a manipulator's mind.