Fractured Families required a serial killer. It was necessary for the plot. The murders were too bizarre to have been committed by a person of ordinary sensibility. I did a lot of research on this subject and know more about truly evil people than is good for me.
My editor asked if she should use the word "sociopath" or "psychopath" in flap copy. Actually the Diagnostic and Statistical Manual, which is the psychologist's bible, called this kind of deviance "psychopath" until 1958, then the term switched to "sociopath." After 1968, information was classified under the heading of Anti-Social Personality Disorder.
For publicity purposes, we settled on using the word "psychopath" because the term is more familiar.
My home state of Kansas has had its fair share of serial killers. Lizzie Borden was famous, of course, and so were the Bloody Benders.
Recently I listened to an old, old book (1830s) The Count of Monte Cristo, on an audio recording. Narrated by John Lee it was the one of the best narrations I've ever heard. A classic tale of revenge, it held my interest for hours.
I think betrayal and revenge is are two motivations that are universal.