Just How, And Why, Does One "Graduate" From Killer To.......

If you really want to understand the psychology of it, buy the complete 13 season DVDs of "Criminal Minds", based on the very real Behavioral Analysis Unit of the FBI. The show hired a retired BAU agent as a consultant.

The early seasons' episodes are more analytical and less about character backstory, but binge-watching that for several weeks should get you up to speed. Many episodes include references to actual serial killers and their crimes.

Or you could read the many books written by two of the 'founding fathers' of the BAU, which are mentioned in the Wikipedia page.

Well, the definition of a serial killer is that you kill more than one person in separate incidents, and that there be no clear motive other than compulsion. Also there might be a continuous Modus Operandi or Method of Operating, though that is not a prerequisite. Your above mentioned Night Stalker had many different MO.

A hit man, though he kills many people, is generally not thought to be a serial killer, because he kills for money, not from compulsion. 

There also seems to be a strong link to mental illness, though again, that is not a prerequisite.

It is my belief that a person who becomes a serial killer was always going to become a serial killer. Whatever is wrong in their head was always already there even before they first killed. It only gets worse after they start doing so. I do not think they can be cured, and the only way to stop them is to incarcerate them for life.

I've said for years if people want to know how these people think/work, if they want to know how school shooters think/work, read John E. Douglas's books, it's an education that kills a part of you, but that's a necessary evil to understand just what kind of people are walking around loose in this world, what the red flags are to watch for, and just why they don't react to the stuff that bothers *you*.