What is the conflict and central idea of the book?

You might consider The Pervasiveness of Evil. Each character in And Then There are None must deal with the issue of evil and its pervasiveness in their lives. In their previous lives, these characters had been responsible for the death of another human being in some direct way. Even though they had all found ways to resolve their own involvement in these evil acts, the killings on Indian Island draw them into a world in which evil has full reign. Christie is commenting on an inherent evil nature within human beings. Though each character believes himself to be innocent despite the death he previously caused, the reign of terror on Indian Island is a final indictment on his inherent evil.