In addition to the other names given the "psycho killer" subgenre that I mentioned in Part 1, "serial killer" is often used to designate the subgenre in film. It's true that in this sense "serial killer" and "psycho killer" mean almost the same thing, and the former even implies that "the killer must kill again," to borrow the title of a 1975 giallo film.
However, "psycho killer" is a better term in another respect. Unlike other types of murderous monsters, the psycho killer is separated by being the creation of psychological forces, which in turn stem from his place within the cultural concept of monsters: the sense of *modernity.*
If one dates horror fiction from the rise of the Gothics in the 18th century, then most of the classic monsters are linked to their predecessors in archaic folklore: vampires, werewolves, and demons. Even Mary Shelley's Frankenstein Monster, despite serving as a prototype for the later science-fiction genre, owes some inspiration to the writings of alchemists like Albertus Magnus. Early cinema was a little more devoted to killers who didn't owe that much to archaic lore, such as the 1925 PHANTOM OF THE OPERA, adapted from Leroux's 1910 novel. But even in films, the best known names in horror either sprang from archaic ideas about magic, like Dracula and the Golem, or from an opposite strain: that of proto science fictional concepts like Frankenstein, Doctor Jekyll and Doctor Moreau.
Technically, any of these monstrous menaces might be deemed "serial killers" insofar as they kill a lot of people during their exploits. But vampires, werewolves and the results of mad science don't kill first and foremost out of their psychological maladjustment. Such monsters are the products of magic or mad science, not of bad parenting or broken homes.
The modern idea of "psychology" stems from the late 19th century, and over time provided a new model for motivation. The aforementioned Phantom, of course, pursues a goal of romantic fulfillment in compensation for the physical disfigurement that makes normal life impossible for him. To be sure, the Phantom doesn't actually kill many people in the original novel, but cinematic adaptations have tended to ramp up his kill-count.
Thus I favor "psycho killer" over "serial killer" because the former already implies seriality, and it also implies that the *character* of the killer: that he/she is either a "true psycho" or a "fake psycho." The latter category, of course, would not be conceivable unless audiences could credit the idea of a serial-murdering psycho as a real threat.
Now, in real life "psychos" are considered to be the results of an entirely naturalistic process, However, in fiction, it's possible to have a psycho-killer who conforms to the tropes of the naturalistic, the uncanny or the marvelous. The one common factor they all share is the idea of *modernity.*
Now, whether one is talking about a "modernity" taking place in modern times or in earlier, post-industrial eras, it's as easy to see an uncanny psycho-killer operating with the same basic modus operandi as a naturalistic one, as I pointed out in PENALTY FOR THRESHOLDING. However, what sort of psycho-killer can be both modern and marvelous?
And the answer is "a gho-gho-gho-GHOST!"-- albeit only a very recent revenant.
Thus Freddy Krueger qualifies as a marvelous psycho-killer--
However, the Headless Horseman, were he a real specter, would not, since he's become a thing of legend over the course of years. The same applies to the gigantic spectral helmet that kills a victim at the beginning of THE CASTLE OF OTRANTO.
More examples in Part 3.