Any knife or dagger would be good for this. It doesn’t even have to be a “professional” knife or combat oriented weapon. It can be a kitchen knife, a butcher’s cleaver, a meat hook, a surgeon’s scalpel, anything you want really. If he or she is a savvy serial killer then they’re most likely to use a knife that leaves a minimal amount of forensic evidence. However, unless you’re basing your magic and fantasy setting around a modern 21st century understanding of medicine, detective work, criminal profiling, and forensics, it doesn’t really matter. He’ll use whatever is within the range of he has access to and maybe has special meaning (maybe not), perhaps a knife with an interchangeable handle and one that is easy to clean. It really depends on what type of killer he/she is and since I don’t know the character, the setting, or the type of law enforcement in question it’s really difficult to guess. (I say he because most of the serial killers we know of and profiling circles around are male, but historically there have been several prominent female ones.)
While serial killers have probably existed for as long as humans have, our understanding of their psychology (and even the use of the term “serial killer”) really only dates back to the 1960s-1970s before that they were something of a mystery.
I’d actually step back a moment and look at serial killers. I’m going to pull a passage from Whoever Fights Monsters: My Twenty Years Hunting Serial Killers for the FBI by Robert K. Ressler and Tom Shachtman dealing with the profiling of “The Vampire Killer” aka Richard Trenton Chase. Ressler, arguably, coined the term “serial killer” in the mid-70s, and is one of the originators of modern serial crimes investigation.
Here, in the original (and not entirely grammatical) notes written at the time is how I profiled the probable perpetrator of this terrible crime:
“White male, aged 25-27 years; thin, undernourished appearance. Residence will be extremely slovenly and unkempt and evidence of the crime will be found at the residence. History of mental illness, and will have been involved in use of drugs. Will be a loner who does not associate with either males or females, and will probably spend a great deal of time in his own home, where he lives alone. Unemployed. Possibly receives some form of disability money. If residing with anyone, it would be his parents; however, this is unlikely. No prior military record; high school or college dropout. Probably suffering from one or more forms of paranoid psychosis.”
Though profiling was still in its infancy we had reviewed enough cases of murder to know that sexual homicide — for that’s the category into which this crime fit, even if there was no evidence of a sex act at the crime scene — is usually perpetrated by males, and is usually a intraracial crime, white against white, or black against black. The greatest number of sexual killers are white males in their twenties and thirties; this simple fact allows us to eliminate whole segments of the population when first trying to determine what sort of person has perpetrated one of these heinous crimes. Since this was a white residential area, I felt even more certain that the slayer was a white male.
Now, I made a guess along a great division line that we in the Behavioral Sciences Unit were beginning to formulate, the distinction between killers who displayed a certain logic in what they had done and whose mental processes were, by ordinary standards, not apparently logical— “organized” versus “disorganized” criminals. Looking at the crime-scene photographs and the police reports, it was apparent to me this was not a crime committed by an “organized” killer who stalked his victims, was methodical in how he went about his crimes, and took care to avoid leaving clues to his own identity. No, from the appearance of the crime scene, it was obvious to me that we were dealing with a “disorganized killer” , a person who had a full blown or serious mental illness. To become as crazy as the man who ripped up Terry Wallin is not something that happens overnight. It takes eight to ten years to develop the depth of psychosis that would surface in this apparently senseless killing. Paranoid schizophrenia is usually first manifested in the teenage years. Adding ten years to an inception-of-illness age of about fifteen would put the slayer in the mid-twenties age group. I felt that he wouldn’t be much older for two reasons. First, most sexual killers are under the age of thirty-five. Second, if he was older than later twenties, the illness would have been so overwhelming it would already have resulted in a string of bizarre and unsolved homicides. Nothing as wiles as this had been reported nearby, and the absence of other notable homicides was a clue that this was the first killing of this man, that the killer had probably never taken a life before.
Whoever Fights Monsters, Robert K. Ressler and Tom Shachtman, pg 3-4.
Sir Also Appearing In This Book: Ted Bundy, John Wayne Gacy, David Berkowitz (“Son of Sam”), Charles Manson, Edmund Kemper, Peter Sutcliffe (“The Yorkshire Ripper”), Richard Speck, and Jefferey Dahmer. If you’re planning to write about a serial killer, even a fantasy based one, I recommend reading about what the experts who caught actual serial killers have to say before turning to recent television like Dexter or Hannibal. The book also includes some discussion of the various crime scenes and killing which may provide you with some (admittedly rather gruesome) inspiration.
What kind of killer is your killer? Organized? Disorganized? If we’re discussing someone who routinely uses the same weapons over and over again, I’m going to guess these aren’t crimes of chance. Though whether or not this was the weapon he first began killing with (and holds sentimental value) is probably a question worth thinking about. If it is, then it’s likely a common one that’s valuable to his daily activities.
Is he stable (and capable of holding down a job) or mentally unstable? Why does he kill his victims? In the case of “The Vampire Killer”, he believed the people he was killing were tied to a secret Mafia organization that was poisoning him for his mother. In his trial, he firmly believed this was his chance to out the truth. We know untreated paranoid schizophrenia often results in these sorts of delusions.
Why your killer does and who he targets are going to be much more important than what he performs his killings with. Women? Men? Girls? Boys? Nobles? Merchants? Prostitutes? Religious minorities? Is he punishing his targets for some perceived slight or sin (stalking and killing prostitues because they represent immorality and corruption, coupled with a repressed sexual desire)? Is he trying to save the world? Are his killings just hack jobs or do they have a theme?
It’s all up to you really.
References for Further Reading/Viewing:
Whoever Hunts Monsters by Robert K. Ressler and Tom Shachtman. I’ve already said why this should be on your shelf if you’re writing serial killers, but I’ll say it again: FBI expert and discussion of real case files. When it comes to research: Reality > Fiction.
Seven In this famous thriller, Morgan Freeman and a young Brad Pitt star as two cops chasing down a serial killer who performs crimes based on the seven deadly sins. (Yes, Supernatural fans you can finally learn “what’s in the box” though you may wish you didn’t.)
From Hell by Alan Moore. One of Moore’s lesser known (i.e. less popular than Watchman) works surrounds the investigation into the possible identities of Jack the Ripper. Not only is it very good, it’s also very thorough.