What is the theme in Red Dragon by Thomas Harris?

A major theme of the novel, Red Dragon, is the concept of mental instability and the idea of a human being having the capacity to commit murder. Will Graham is a gifted investigator, who has tracked down two serial killers who both taxed his mental state before he retired from the FBI. The first killer forces Graham to kill him while he holds a young girl in his arms with a knife at her neck. Graham is haunted by the girl's expression of terror, and the fact that it took many bullets to bring down the murderer.