Alright, so you have completed the history of the killer. It was nice and straightforward and required little creativity. This part is pretty much the opposite. Your killer has to think in a way that's consistent and believable. Obviously then your killer has to have a personality.Now ordinarily I prefer to design my killers with the DSM-IV-TR (The Diagnostic and Statistical Manual of Mental Disorders Fourth Edition with Text Revisions), however, it is not absolutely necessary. If you do decide to use the DSM make sure you use the complete diagnostic criteria and that you are familiar with the whichever disorder you chose to use. This will require you to read from the definition of the class of mental disorder to the associated lab findings of the specific disorder. If you do not I would recommend styling your killer after a well known killer or someone you know. There are also many archetypes in literature that you can use to aide you in your designing your killer. When in doubt about a killers personality always defer back to the personality. Our life experiences make us who we are. The same is absolutely true for killers.
Killers in history you may want to style your killer after:
*Ted Bundy
*Gary Ridgway
*Jeffrey Dahmer
*Ed Gein
*Eddmund Kemper
*Andrei Chikitilo (would make for an excellent horror story)
*The Hillside Stranglers
*Jack the Ripper (Check out the Jack the Ripper profile I have uploaded *wink wink*)
*The Zodiac Killer
*The offender in the Black Dahlia Murder
*Timothy McVeigh (Oklahoma Bomber)
*Ted Kaczynski
*The killer in Se7en (fictitious)
*Hannibal Lector (fictitious)
*George Metesky, The Mad Bomber (Ok so he technically didn't kill or injure anybody but he was the first direct use of a criminal profile that lead to a criminals arrest in the USA)