Soviet serial killer Andrei Chikatilo just may be the sickest serial killer I've ever read about. I wrote a paper on him in my Homicide Investigation class and actually had nightmares. It wasn't an easy paper to write, let me tell ya.
Yes. I think he's by far the most prolific of all serial killers. I forget the author's name, but there is a good book about him, called "Hunting the Devil."
