Realized that motive is the core of every mystery novel

Finished reading The Moonflower Murders and immediately started reading Death on the Nile. I liked the first book, but I LOVE the second one. My stereotype of Agatha Christie is that she was one of the world's most inventive storytellers, but her characters were cardboard (I've only read Murder on the Orient Express and The Murder of Roger Ackroyd). But…