There are a few shows out where the antagonist is a serial killer type that has a network of followers: The Mentalist and The Following most prominently, and Elementary partially. Is there a documented real world basis for these 'cults', maybe even one from which those shows drew inspiration?
I am not aware of specific cult followings that these shows draw from, but the lore and allure of serial killers and their followers has been a long established fact.
Some of the more famous serial killers that have cults include (But are not limited to):
Charles Manson - The Manson Family were a bunch of women that followed him as a guru, and whom he sent out (And some on their own) to commit murders. The wiki details on this start about 20% down the page.
Jeffrey Dahmer, Ted Bundy and Richard Ramirez, while they didn't have cult followers in the manner of Manson, did attract many (in some cases hundreds) female followers, many of whom would visit in prison, send letters, gifts and promises of adoration/undying love. (The phenomenon is known as Hybristophobia).
In addition to this, there are many "followers" of serial killers that know every detail of their lives and cases, and often also collect artifacts from their lives (This industry has the name murderbilia). While these are not cult followers in the Manson sense, it does demonstrate the fascination that exists for serial killers. Quite a few serial killers have ended up marrying some of their fans, either while in prison or after release.
There are copycat crimes:
Some real life examples can be found in the link above and :
I don't know when was the copycat killers introduced in fiction, but I found a movie from 1969 that deals with the theme:
There have been also murderous cults like the Thuggee: