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?
There are copycat crimes:
A copycat crime is a criminal act that is modelled or inspired by a previous crime that has been reported in the media or described in fiction. http://en.wikipedia.org/wiki/Copycat_crime
Copycat killers are a very common plot twist in police dramas, movies, and mystery novels where the plot involves serial killings. The copycat may occasionally serve as a Red Herring - the detectives think they've caught the killer, but it turns out he was just a copycat and there's still a serial killer on the loose. (Although, in Real Life, investigators of a serial killer often keep some details from being released to the public in order to tell a possible copycat apart from the main suspect, and some realistic crime novels mention this as well.) Alternatively they could be tempted to doubt they have the right guy because the crimes haven't stopped.WARNING TVTROPES WILL RUIN YOUR LIFE
Some real life examples can be found in the link above and :
- Ripper copycat is found guilty of murder
- 10 Notable Copycat Killers
- List of alleged Natural Born Killers copycat crimes
I don't know when was the copycat killers introduced in fiction, but I found a movie from 1969 that deals with the theme:
Night After Night After Night (1969) was a low-budget production that cast a high court judge (played by Jack May) as a demented copycat Ripper who attacks prostitutes in London's Soho Jack the Ripper in fiction
There have been also murderous cults like the Thuggee:
Thuggee is described as a cult of people engaged in mass murder. The modus operandi was to join a caravan and become accepted as bona-fide travellers themselves. The Thugs would need to delay any attack until their fellow travellers had dropped the initial wariness of the newcomers and had been lulled into a false sense of security, gaining their trust. Once the travellers had allowed the Thugs to join them and disperse amongst them - a task which might sometimes, depending on the size of the target group, require accompaniment for hundreds of miles - the Thugs would wait for a suitable place and time before killing and robbing them. Thuggee
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.