I once read a book by a psychologist who also wrote thriller novels about serial killers. Even he, who's professional life revolved around entertaining people with stories of serial killers, admitted that real life serial killers were usually terrifically boring and often rather pathetic people. From what I can tell, most of them are just sex killers anyway. But in stories, they're transformed and glorified as some sort of elite order of vampire knights, with their own codenames and trademark weapons and a superhuman competence, toying with the mere mortals they hide among. It's kind of like how in horror movies and superhero stories nuclear radiation transforms you into into the hulk, as opposed to riddling you with tumors and leaving you trapped in a hospital bed.
I've been watching the Hannibal show a bit lately and it strikes me as a good example of this; they depict the world as a place where serial killers are just boiling out of the woodwork, launching a new assault against society every week as they swap notes with their "brothers" on their "tradecraft". It's fine there, because they stylize it enough where the show's universe seems like a bad-dream version of real life and can thus freely take liberties with such things, but Hannibal's general take on serial killers is hardly unique and is elsewhere (from what I can tell) often portrayed as "frighteningly realistic", as opposed to a heavily embellished aspect of a nightmarish contemporary fantasy.
My point is that fictional serial killers may be interesting, but that's because heavily liberties are taken, and it's important to remember the "fictional" part.