Solving the writer's knowledge

Every time you find a mystery from before the 1980s or so that features a serial killer, it turns into a special type of "solving the writer" where you are guessing whether the writer is aware how serial killers work, and the earlier the story, the less likely that they are. An episode of Morse comes to mind, but I'm reading a Mrs Bradley mystery now that has me on the edge of my seat for signs that anyone in the story is even slightly aware of serial killer patterns. 😂 So far the only sign they might be is that some of the characters say that serial killing means a maniac or crazy person, without further elaboration.