Camus' focus in The Guest, as far as moral message or theme goes, ultimately points back to Existentialism. That is, at the core of human existence is its fundamental futility. Everyone dies eventually, and therefore life is, for all intents and purposes, meaningless. In The Guest, Camus continues this outlook and nudges the reader to think about choices and consequences and the need to live with the choices one makes. Read the story with these principles in mind. There are many sites on the Internet where you can read more about Camus and the Existentialists.