Book excerpt: Sunday Times bestselling author Christopher Berry-Dee is the man who talks to serial killers. A world-renowned investigative criminologist, he has gained the trust of murderers across the world, entered their high security prisons, and discussed in detail their shocking crimes. Berry-Dee now delves into the mind of perhaps the most sadistic and psychopathic killer of all time. Between 1978 and 1991, Jeffrey Dahmer murdered and dismembered seventeen boys and men. But he is most notorious for what happened to his victims after their grisly deaths and the shocking depravity that led to Dahmer being dubbed the 'Milwaukee Cannibal'. Using his long experience and psychological expertise, Berry-Dee seeks to understand the motivation, the amoral urges and the merciless horror behind Dahmer's inhuman behavior: what could make a man do this?