This book examines what makes someone an evil person and how evil people are different from merely bad people. Rather than focusing on the "problem of evil" that occupies philosophers of religion, Barry looks instead to moral psychology—the intersection of ethics and psychology. He provides both a philosophical account of what evil people are like and considers the implications of that account for social, legal, and criminal institutions. He also engages in traditional philosophical reasoning strongly informed by psychological research, especially abnormal and social psychology.

In response to the popularity of phrases like "the axis of evil" and the ease with which politicians and others describe their opponents as "evil," Barry sets out to make clear just what it is to be an evil person.