At the risk of sounding like the opening to an episode of Castle, there are three kinds of people who sit around thinking about the psychological makeup of murderers--profilers, cops, and mystery writers. Unlike profilers and cops, I can rest assured that the murderer will be caught and that I am never wrong with what events created him. Last weekend, I put aside the project I was working on to start a new one and I always start with the characters--and in this case, that includes at least one murderer.
Actually the process of creating a character, regardless if they are a protagonist, antagonist, the nice granny character or even a murderer, is basically the same. Every writer tends to create their characters differently, but the purpose of the process is to let the writer get into the head, or heads, of their characters. When the writer successfully does this, he can draw the reader in to those same characters' heads.
There are two psychological theories that I use to create the killers in my stories (actually to be truthful, I use it to create my protagonist and other characters as well):
1. Meyers-Briggs Personality Type Indicator:
I first ran into this theory when I was working on my Masters in Leadership degree. The Meyers-Briggs (MBTI) is a theory of psychologically typing that is built upon the works of Carl Jung. MBTI looks at the way a person takes in, analyzes, and react to things and data in this world.
Each personality is composed of a choice between four pairs of elements that sit on a spectrum:
Extroversion or Introversion
Sensing or Intuition
Thinking or Feeling
Perception or Judgmental
My main textbook on this subject is a "one of a kind" self-published by the professor given only to his students, but there are plenty of good books on the subject.
2. The Lucifer Effect:
Many people think people are born a certain way while others think it is caused by the environment. Regardless of which side may be right, the exploration of the environmental role is much more interesting from a writing point of view. I use this psychological theory for both my protagonist and the antagonist--especially any killers I create. The Lucifer Effect is also known as the Stanford prison experiment. The original experiment took a group of normal people and split them in two groups: prisoners and prison guards. What the experiment discovered was that there are certain roles and events that can change our personality and transform the person we thought we were. With this idea, I explore what would cause the killer to transform from an ordinary person into one that kills or what events can push someone to the edge and can they pull back from it.
To learn more about the Lucifer effect, I recommend a book by the same man who ran the experiment: The Lucifer Effect: Understanding How Good People Turn Evil by Philip G. Zimbardo