Its a morbid fascination. These are people who are very...unique, I guess. And getting a somewhat intimate, behind-the-scenes view...or any view for that matter, that can offer insight into what makes people like this tick. How they were able to gaslight those around them, what drove them to do commit such heinous acts.
Maybe its the modern day version of the freak show from back in the day.
As far as watching the reenactment of murders, I'm not sure anyone really WANTS to see that. I mean, its likely not what most of us tuned in for and IIRC, I don't think they really showed much at all of that in this film. My bet is that people tune in more for the story of the murderer himself, but yeah I've thought of that too...hoping that the families of the deceased stay away from movies like this, lest they see a reenactment of their loved one's murder. That'd be some horrific stuff right there.
There have been serial killers recorded since the beginning of psychology.
I have a book from the 1800s, a psychology book, that explains many of them. They're pretty typical with almost all of the same causes. So, these people aren't unique at all. My work with them have proven that if you meet one, you've met them all. But, the media presents them as a kind of porn for weird people to enjoy.
If you look up the definition of porn, you will see.