Amongst the standard protagonists of horror RPGs are monsters, alien entities, nameless gods, death cults and foul demons. But there is another protagonist who can be useful to a GM seeking to motivate her players; the serial killer. BEWARE: this topic does require a certain level of maturity on the part of the GM and players.
In a historical context, the phrase "serial killer" was not uttered until the late 1940"s or early 1950"s. Until the early 1970"s police forces were not suitably prepared for criminals of this nature and unlike today, the public were completely ignorant of the motives of serial killers. In a modern campaign there are plenty of references for PCs to work from. For campaigns set in the 1890"s, 1920"s and 1930"s, the lack of everyday reference implies "mystery".
Sadly, there are many historical cases to draw upon for RPG inspiration. The most obvious candidate is Jack The Ripper, terror of gaslit London, but he is not the best example for a GM. Those dubious honours go to modern, well-chronicled killers such as Peter Sutcliffe and Jeffrey Dahmer. There are also many fictional examples, from Jason to Hannibal Lecter, who may serve as templates. Whilst there are examples of serial killers in published horror RPG scenarios, most are inappropriate in this context. One notable exception is the scenario "Death In The Post" which appears in the Cthulhu sourcebook "Green and Pleasant Land".
Profile of an average(?) serial killer: Typically male, mid 20"s to late 40"s, above average IQ, good education, probably an underachiever, emotionally repressed, perhaps sociopathic. Serial killers often seem perfectly normal to those around them and function well in daily life. They usually operate alone and can be both methodical and audacious. Their motivations vary wildly but many seem to involve sexual compulsion. It is very common for serial killers to commit the same crime repeatedly (e.g. murder by strangulation) whilst refining the crime each time. It is also common for modern serial killers to become more sloppy as their compulsions take precedence over cunning and caution.
Although there are those who don"t fit this model, it is suitable for our purposes. Let"s examine our protagonist in game and campaign contexts.
The primary problem with a serial killer as a scenario focus is their humanity(?). With no supernatural powers they are as vulnerable to damage as are their victims. Unless the GM plans on emulating the recuperative powers of "The Hitcher" then a scenario involving a serial killer will not necessary end with a protracted violent climax. Serial killers are more suited to atmospheric mystery scenarios where the protagonist kills offstage and leaves evidence and/or clues to be found later. This lack of a visible enemy can make for a lot of tension if handled properly.
The first requirement is a comprehensive character background for the killer. This should include physical description, everyday habits, major psychoses/neuroses and a complete breakdown of the modus operandi of and motivation for murdering.
Focussing a scenario entirely on the hunt for the killer can make for a short scenario. One successfully way to use a serial killer is within an ongoing scenario, perhaps based around a location frequented by the characters, or a character hobby. It is far more atmospheric to have the killer appear across multiple scenarios, intertwined with supernatural doings. Leave evidence and clues which do not reveal patterns until many such items have been assembled. This means that the GM must artificially arrange to have evidence obscured or missing from crime scenes until it is time for the shoe to drop. It is also vital to prepare a timeline of previous events, alibis and a proposed timeline of events to come.
Within such a mini-campaign, the killer can have many faces. Here are two examples:
The (male) killer was abused as a child and therefore murders men who remind him of the abuser, his father, a Baptist minister. The killer is a devout Christian who works as a librarian at a large reference library. The quiet and refinement of the library suits his introverted, bookish nature. The killer selects his victims from amongst the visitors to the library, perhaps those who browse books on restricted topics such as occultism. One of the PCs is a dead ringer for the father and becomes a potential victim when searching for forbidden lore. The killer attacks his victims in their home with a blunt object (paperweight). He uses the pretext of examining a library book which the victim has borrowed. He lays the corpse out "in state" on their bed as if prepared by a funeral director, a twisted mark of respect for the dead. Clues can include overturned books, library slips, the physical similarity of the victims.
The (female) killer is a classical Bohemian artist who paints nudes. Daughter of a titled English couple, she is estranged from her family because she lived amongst Parisian artists, experimented with drugs, became pregnant and had an abortion. Each victim was at one point her lover, whom she painted as a nude. She suffers from a jealous-obsessive disorder causing her to become extremely possessive over her lovers and she has a very low sense of self worth. If the lover ignores or accepts her behaviour, he or she is soon dumped as they are not worthy to be immortalised on canvas. Should a lover come to resent her possessiveness, that lover is killed. The killer attacks victims in her paint-spattered studio apartment using a sharpened palette knife, usually when the victim is posing in the nude. Victims are dumped at night in parks or squares containing commissioned sculptures. Clues include the bloodstained sheets in which the victim was draped, paint splatters, evidence of sexual intercourse, stab wounds (left handed, females tend to stab underhand), paintings she has sold to galleries.
A serial killer may be used in many ways other than as a straight killer. The first example above could be a regular and useful NPC in a "Call of Cthulhu" campaign for many months until the compulsion to kill a PC is triggered. The second example could be the lover of an NPC cultist whom the players wish to question. When the cultist ends up dead the PCs may become suspects!
It is important to realise that the GM must roleplay the killer honestly and well, hence the need for the detailed character background and tineline. Serial killers should not be common in a horror campaign unless they are its focus e.g. a campaign based on an FBI crimes unit. However, one well-presented serial killer will induce far more player nostalgia than a swarm of tentacled monsters.