The narrator of the story did the murder.
Up for Grabs. So here you are, reading your Whoddunit. Fascinating book. It's written from the point of view of some random guy who has gotten very close to the detective. You like the guy, he seems like your average dude (it's usually a dude), with a sense of humor. The detective never tells him anything, darn him. Finally, you reach the last chapter. The Reveal is about to be made! Holding your breath, you run through the pages, read, read... and the murderer is... ... the narrator? Alternatively, the story may not be a whoddunit. It could be the confessions of a criminal, a diary, or something similar. In such a case, you would (probably) know the murderer's identity from the beginning. This could use a better description, but I hope you get the idea. Needs More Examples. Spoilers ahead, obviously. Examples: