Murder Is Easy 

by Agatha Christie

Recently retired, police detective Luke Fitzwilliam finds himself on a train, sitting next to Miss Pinkerton. She is on her way to Scotland Yard as she is convinced that her village has a serial murderer.
The conversation gets around to how easy it is to identify a murderer:
Luke had the grace to blush. ‘Well,’ he said. ‘So many murders! Rather hard to do a lot of murders and get away with it, eh?’ Miss Pinkerton shook her head. She said earnestly: ‘No, no, my dear boy, that’s where you’re wrong. It’s very easy to kill – so long as no one suspects you. And you see, the person in question is just the last person any one would suspect!’

Fun read - and a nice twist on whodunnit.