
People of the Lie is a psychiatrist's attempt to fuse science and religion by explaining the phenomenon of evil people with psychiatry. In a sentence, it was an interesting read, but I believe he was ultimately wrong.
According to Peck, evil is basically a kind of disease: malignant narcissism. The evil person is one who refuses to submit one's will to a higher power, be it God or some other virtue or discipline. Believing they are right and refusing to accept otherwise, they force others to live according to their worldview. Because they believe they are right, they will lie to themselves and others in order to convince others of their virtue (hence the title of the book, People of the Lie).
While using psychiatry terms to describe evil can be useful, I think that equating evil with narcissism is a mistake. First and foremost, narcissism is sometimes a medical condition, and other times a choice. Evil doesn't work that way. Evil isn't a medical condition that afflicts people. Second, calling evil a disease possibly suggests a genetic element, which is just wrong. The dichotomy of good and evil isn't about one's genetic structure, nor should it be.
As I said before, People of the Lie was an interesting read, but he simply wasn't right. It was fairly well written. I feel that it did dip too far into the occult for a nonfiction book, but only occasionally.