How murder of Roger ackroyd diagrasses from the classic mystery novels ?

Considered by many to be Christie’s masterpiece, the novel is nonetheless controversial for it’s stunning ending. Christie admitted she got the idea for the ending from her brother-in-law, James Watts, who mused on a detective novel in which the criminal turns out to be the “Dr. Watson” character, referring to Watson’s position in Sir Arthur Conan Doyle’s Sherlock Holmes series as the companion and chronicler of the brilliant detective.

In his 1927 essay "The Great Detective Stories”, from The Art of the Mystery Story edited by Howard Haycraft, Willard Huntington Wright complains, “The trick played on the reader in The Murder of Roger Ackroyd is hardly a legitimate device of the detective-story writer; and while Poirot's work in this book is at times capable, the effect is nullified by the dénouement.” For many, Christie’s twist ending was considered a mutilation of the detective story genre, where certain truths – of the brilliant detective and the objective narrator, for example – are expected to be able to be taken for granted.