In the old days it was believed that there were only 2 reasons a serial offender stopped , this goes for rapists etc...
And that was that they were either dead or incarcerated for something else. As time went on and the field of criminal studies progressed we began to learn more and more about crime and those responsible for it
Contrary to what we thought we once knew about serial crime, there were MANY reasons these repeat offenders stopped doing their actions
AS noted above incarceration, and death would certainly hamper their ability to continue , however we know know thats only 2 of many.
AS we learned (Dennis Rader or Lonnie Franklin for example) stopped killing for a variety of reasons, Rader felt LE was getting too close to him, Franklin simply "lost the urge" at times and found other ways to satisfy his criminal psyche.
The criminal lifestyle itself is fraught with danger, we've seen killers who have become physically or mentally disabled, via trauma, disease or otherwise , we've seen them abscond to other parts of the world, sometimes, they are relocated for work, or due to military service, etc....sometimes in the new location they continue unbeknownst to us , the Highway Serial Killing Initiative for example exemplifies this , we've even on rare occasions seen a change of "heart" if you will , but ever so exceedingly rare enough to turn themselves, in , so I tend to call BS on that.
Some find other "outlets", such as drug addiction, some find religion, or so they claim, some have a complete lifestyle change, such as coming into money.
Some may change what they do slightly, for ex going from serial murder to rape, however the usual progression is the other way around
So why did he disappear? As I said above, it couldve been any one of a hundred reasons, in the case, of this particular criminal I feel he knew he was very close to getting caught, and stopped.
One particularly common thing I find about these types who taunt police, the "Letter writers" is despite all their tough talk and attempts to be scary, they are usually very scared/ paranoid individuals themselves.
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The taunting of authority figures, is often a key to what you are dealing with as a severely inadequate, psychologically weak individual to begin with . It basically screams "I couldn't accomplish anything in the real world, so I created my own, and I'm god in it" , its akin to the child who "invents his own" game, because he cant win playing real ones, except this is an adult with severe disorders.
This is the role of fantasy in the killers mind
A better path to pursue in order to find out who this was would be , "Why did he start"? This wasn't some epiphany he had one day and decided to start killing, theres a path to this, look for the crimes leading up to it.