opportunity to lead a normal life. and so this is a wonderful story. >>> he set face to face with killers who were the darkest, the btk killer, tapping into their minds to understand what motivates them to murder. the architect of american criminal profiling takes us inside the mind of two serial killers and draws out two disturbing confessions. >> would you have kept going? >> yeah. i felt really comfortable about killing. i mean, killing for me is just like second nature now. >> joseph candro, a cold-blooded killer with a sick obsession. >> she had no idea that would be her last day alive on this plan etd. >> donald harvey, a hospital care giver with a deadly secret. >> do you consider yourself a society yo path. >> that's what most consider me. >> what do you think? >> i have been rehabilitated. i'm ready to go back to the streets. >>> i'm john douglas. for close to 25 years i ran the fbi's behavior science unit. as a criminal from filer my job so is to help criminal investigators...