Posting to the pre-hijacked topic:

Although these people certainly aren't of a cookie-cutter psychological makeup, they do bear certain similar characteristics. A recent book that's been written on the subject is My Life among the Serial Killers: Inside the Minds of the World's Most Dangerous Killers, by Helen Morrison, M.D., & Harold Goldberg.

The one thing that still eludes psychologists and sociologists is the serial killer's initial motivation. It's known that they are experimenters from an early age, with considerable curiosity but almost completely without empathy for the subjects of their experiments. What initially triggers the development of these characteristics, and the others that follow to make up the serial killer's psyche, is unknown.

As more is known about Joseph Edward Duncan III's most recent crimes, the characteristics of a serial killer are falling into place one by one.