Looking for books on the psychology of serial killers

And JUST the psychology of serial killers. I don't care about the details of the crimes or the police investigations or the criminal profiling or their biographies. I'm looking for something that is purely focused on examining the psychodynamics behind why they do what they do and how they evolved this way. I'm not interested in the criminal angle of it all. I've been scouring the internet and I can't find a single book that fits the bill!