Who murdered the mystery movie?

Agatha Christie is often credited with originating the "summation gathering," which appeared in her very first novel, The Mysterious Affair at Styles. Although considered a somewhat stodgy device today, it still works to convey the sense that everyone is guilty of something, so everyone in the vicinity of the crime gets to squirm under the eye of suspicion.
(The pinnacle of this was in her book And Then There Were None, in which everyone really is guilty. Not too many people left for the summation gathering in that one, though).

By the way--Happy Birthday to Agatha Christie, Queen of Crime, surfing pioneer, and cougar. In honor of Dame Christie, write a poison pen letter to a friend.