This step is 100% essential to designing your killer. This is the killers background information that makes him or her who they are. This history must take the killer from infancy to whenever the story takes place, but you only need to include events that are significant to the killer. Here are some examples.
*Was he or she ever abused?
*If abuse was present, was it physical or sexual?
*If sexual what kind of person abused him or her? (alcoholic dad making daughter into surrogate wife after actual wife dies, sadistic pedophile, catholic priest, etc)
*If physical, how extreme was the abuse, how did it happen, who abused him or her, what were the circumstances, and when did it take place?
*Was he or she bullied?
*If he or she was bullied how severe was the bullying and what kind of person bullied him or her?
*Were there any consensual or non-consensual sexual relationships within the family? (This includes the presence or absence of an Oedipal complex, an Electra complex, or any type of incest)
*Were there any major lies told to him or her? (For example Ted Bundy was told that his mother was his sister and his grandmother was his mother. He wasn't ever told by his family this, but he suspected it his entire life. He eventually found out by requesting his birth certificate from city hall.)
*Did the killer witness any traumatic events at an early age?
*Was the killer abandoned by a parent or another loved one?
*Did the killer ever have his hear broken? (Like Bundy and Stephanie Brooks)
*What is the economic status of the killer?
*Has the killer ever lost anyone important to him or her? If so, who and how?
*Anything else that had a profound psychological impact on the killer.
This part is rudimentary to creating a realistic killer. It will help you make a better plot, have him have more consistent behavior, and will even help you create his personality. Now I recommend not letting the reader know everything in the killers history due to the killers inhibitions. Most killers (and people) in history hold back telling what is most important to them, and often try to hide it.