I like John Douglas but he's very much a dinosaur at this point; I don't think he's caught up with modern criminal behavior profiling. It's not to say he doesn't have good instincts, but sometimes people just don't keep up. It's changing rapidly, especially with genetic genealogy. The notion that no one just goes out and commits a crime is falling by the way-side. Cases of murders by a stranger have a pretty low solve rate so it's a small, and probably homogenous sample size that profilers work from. He's correct in that I don't think people just wake up one day and decide to murder having never thought about it before. But I do think that people spend a lot of time fantasizing about horrible things. So maybe he got up that day and decided today was the day he was going to do the thing he'd been thinking about and planning for 15 years. We really just don't know at this point.
One thing we do know is that people's first murders are often not very well planned or executed - which is why most people get caught. Frequently they are impulsive and opportunistic, with signs of overkill.