Check out the book: Why We Love Serial Killers by Scott Brown. He does an amazing job explaining the psychology behind it all from communities like us, to the media's love for them, to the collection of 'murderablia' and more. The author also features interviews with Dennis Rader and David Berkowitz to show an interesting juxtaposition between a remorseful vs remorseless killer; one paying penance and one who would kill again.
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The book is called "Why We Love Serial Killers: The Curious Appeal of the World's Most Savage Murderers" by Scott Bonn and here is the amazon link to buy it! https://www.amazon.com/Why-Love-Serial-Killers-Murderers/dp/1629144320
I might suggest going a different way. In order to get into the mind of a serial killer, it might be helpful to first understand some of the psychology behind the fascination. This book gives an excellent overview of not only why we as people get very interested in serial murder, but also dives into a lot of other areas that would be essential in authoring a story, in my opinion. I write as well and was glad to have discovered this book. You also might want to check out Ted Bundy: Conversations with a Killer, as this book offers a unique look not only into Bundy's mind, but also into the essence of serial murder in general.