I FIRST learned of Peter Stumpf, the werewolf of Bedburg, when I was 12 years old and off sick from school with scarlet fever. It was 1982 – November to be precise, as I remember Channel 4 launching the same day that Scarlet Fever laid me low.

When I wasn't watching what the new channel had to offer – Walter, The Comic Strip Presents, Countdown, Brookside and odd foreign films – I was ploughing my way through the kind of books most boys read at that age: The Thirty-Nine Steps, King Solomon's Mines, The Lost World, Sherlock Holmes, Treasure Island. Then I turned my attention to my mum's Dennis Wheatley collection – scaring myself silly with The Devil Rides Out, To The Devil A Daughter, The Satanist, They Used Dark Forces, The Haunting Of Toby Jugg.

When I ran out of books, I asked my granny to go to the library and "get me something even weirder than Dennis Wheatley". She must have done her research well – or else had the assistance of quite a dark librarian – as she came back with strange forgotten books by an Edwardian occultist, folklorist and antiquarian called Montague Summers – A History Of Witchcraft And Demonology; The Vampire: His Kith And Kin; and The Werewolf In Lore And Legend. It was this last work which set me on the path to write my latest novel, The Wolf Trial – for Montague Summers told over just a few pages a true story from Germany in the 1500s of a man called Peter Stumpf, known to history as the Werewolf of Bedburg, and now the central figure in my novel.

Summers recounted what little was known of Stumpf, culled from pamphlets and woodcuts – contemporary, almost tabloid accounts of his trial and execution as a werewolf. One line, quoted from a pamphlet, sticks with me. Stumpf, we are told, was "straight transfourmed into the likenes of a greedy devouring Woolf, strong and mighty, with eyes great and large, which in the night sparkeled like unto brandes of fire, a mouth great and wide, with most sharpe and cruell teeth, A huge body, and mightye pawes".

In these pamphlets there is no doubt in the writers' minds, or in the minds of the men who sat in judgement at the trial, that Stumpf was a werewolf, an agent of Satan. Even as a 12-year-old with a 104-degree temperature and tastes that were increasingly Hammer Film, I knew that werewolves didn't exist (despite how much I hoped they really did). When I read the account of Peter Stumpf, and his life, crimes, trial and execution, I was immediately aware – as any sane 20th-century human would be – that what I was reading was an account of a serial killer. Peter Sutcliffe had been convicted as the Yorkshire Ripper the year before (the case had horrified and captivated me), and I'd read about the crimes of Ed Gein, the cannibal murderer who inspired the book and the film, Psycho. To me, there wasn't much difference in what Stumpf had done compared to the crimes of Sutcliffe and Gein, except for a space of 400 years or so.

My tiny pre-teen brain was slightly blown: the world's first documented serial killer had been put on trial as a werewolf, not as a human being. In the medieval minds of the German constables and soldiers who captured Stumpf in the 16th century, and the prosecutors and priests who put him before the courts, there was no concept of serial killing. The authorities – the courts and the church – had no way of explaining Stumpf's horrendous crimes without turning to religion and the supernatural. He must be a werewolf, they thought – for Peter Stumpf had eaten his victims. The pamphleteer tells us so: "He eate their hartes panting hotte and rawe, which he accounted dainty morsells and best agreeing to his Appetite."

So I finished Montague Summers and then picked up Dracula – and that was that, I was destined to a lifelong love affair with the darkest of books, to such an extent that I would end up writing them as I grew older. Stumpf continued to fascinate me and in a way he was my entree to medieval literature, which I studied at university. The more I studied that period, the more I understood why ordinary people of the time saw Stumpf as a werewolf, not just a type of very violent and twisted human being. There was only God before the Enlightenment – no real science, no sociology, no psychology or criminology – and the people of the medieval period lived with their eyes constantly cast down, under the control of God, superstition and the church; they could not look up and challenge anything from the point of view of bare humanism. That way of thinking had not yet been invented.

After leaving university, I worked for many years as a crime reporter and often, when covering a particularly frightful killing, I would wonder to myself: if this had happened 400 years ago, what would readers have thought then? Would they have seen the young boys who killed James Bulger as imps sent from hell and burned them at the stake? Would Myra Hindley have been the witch from Hansel and Gretel?

So, what do we know about the real Peter Stumpf? Very little, but what we do know reads like a textbook study of serial murder. In fact, in some ways he reminds me of Dennis Radar, The BTK Killer – BTK standing for Bind, Torture, Kill – a relatively successful suburban man, loved by his family, who would annihilate entire households in one night for sadistic pleasure, for hatred of the rest of humanity.

In fact, we know so little about Peter Stumpf, that one troubling thought has occurred to me many times over the years: the more I have read and researched his story, the more I have come to doubt that Peter Stumpf was even his real name.

To students of werewolf folklore, his name is just too perfect. It's like finding a real-life medieval vampire case (and those did happen, but that's another story) in which the monster is called Peter Fangs. Let me explain: one of the great tropes of werewolf lore, is the case of the werewolf who is overcome at night by village guards but escapes after its front paw is cut off. In the many variations of the story, the leader of the town guard wakes up in the morning to find his wife moaning with a severed hand, or the wife of the town mayor finds her husband bleeding on the bedroom floor, his arm ending in a "stump". It was they who were the werewolf all along. If you have basic first-year German, you'll probably know that Stumpf is German for stump.

In fact, there is some evidence that Stumpf might really have been called Abal or Abil or Ubel Griswold – now, there is a name to conjure the Devil if ever I heard one. He lived in the mid to late 1500s – a period during which Germany was recovering from intense civil and religious warfare and revolution – in a town called Bedburg. By the time of his capture he was a wealthy farmer, and a widower, with two children, and a lover called Katharina Trump.

When he was captured, Stumpf claimed that the Devil had given him a "magical girdle" which allowed him to shape-shift into a wolf. Why would he say such a thing? Perhaps he was mad, or perhaps the time he spent on the rack accounted for his fantasy. He confessed to being an "insatiable bloodsucker" – the pamphleteers of the time almost writing the playbook for our sensationalist modern press – who killed animals, men, women, children, foetuses in the womb, anything living that Stumpf could get his hands on, mutilate and devour. It was also claimed that he murdered his own son and ate his brains in local woods.

Stumpf also admitted – or made up as his finger nails were ripped out and his limbs dislocated – that he had "mated" with a succubus (a female demon) sent to him from the Devil in hell, and that he had an incestuous relationship with his daughter.

Stumpf's crimes were such that his entire family would be sentenced to die with him. If his "concubine" Trump had slept with Stumpf, then hadn't she had sex with a werewolf in league with the Devil? Wasn't his daughter Beele (thought to be short for Sybil, but with a shade of Beelzebub in there too I think) the offspring of a wolf, contaminated by wolf-blood, who had also had sex with her father? They all had to die – and in the most appalling way imaginable.

Stumpf's lover and his child were flayed, strangled and burned. Stumpf had the flesh torn from his body with red hot pincers, he was broken on the wheel – a punishment which crushed every bone in the body as iron clubs were used to beat the victim strapped to an upright cartwheel – then beheaded and burned.

There are some medieval conspiracy theories surrounding the magic girdle that allowed Stumpf to transform into a wolf. Not only did Stumpf admit having the girdle, but the soldiers who gave chase to him in "wooluishe likenes" claimed they saw the wolf throw off the shape-shifting girdle and change back into a man just before their dogs brought him down and they captured him.

Now, it is all well and good for a man on the rack to say he is a werewolf in order to stop his inquisitor torturing him, and to admit what he is being told to admit, and it is all well and good for the medieval mind to falter when it comes to realising man's cruelty comes from within man, not from without and the Devil. But why would agents of the state claim to have seen Stumpf change back from a wolf into a man when we know incontrovertibly that this did not happen (unless you believe in werewolves too)? The answer is simple: politics.

Stumpf lived in a town which was Protestant following the bloodshed and chaos of the Reformation, but which was under the rule of Catholic Lords. It is not hard to see how a smart and ambitious anti-Lutheran nobleman, or state prosecutor or powerful bishop, might exploit the case of a horrific serial killer for political ends in such a febrile atmosphere. At the time, the subtext of the case might well have been one of: "Good God, look how those Protestants live. Open to the Devil's works, prey to evil, at risk of damnation. They even have werewolves in their midst."

As an aside, Stumpf's trial was attended by some of the most powerful political and religious leaders of the day. There were many other supernatural trials at around this time, and none had such a prestigious gallery.

After his death, Stumpf slipped from public consciousness – though his life and crimes still echo in the werewolf myth today, as we see with the symbolism of his name. When it comes to understanding this echo, we need to turn to an ancient school of study called Euhemerism. It's mentioned by Plato in a passage on Socrates searching to find out the real roots of mythology – for that is what Euhemerism is: an attempt to peel back the skin on a myth to find the tiny nugget of truth within. The best example is the Centaur. Did the first people to see a man mounted on a horse think that man and horse were one? And those Centaurs were notorious for rape and murder – might not the first Bronze Age clan to break and tame a horse have used it for warfare and abduction?

Remember those tales of "real" vampire cases I mentioned earlier. The pamphlets of Moravia and Bohemia in the late Middle Ages are replete with such apparently "true" tales. Folklorists now think they might be down to ergotism – a form of poisoning involving mould-contaminated bread, which has the same effect on the mind as LSD – except that in these vampire cases, everyone in an entire village ate the infected bread and the whole community fell victim to mass hallucinations.

Stumpf, to this end, is a symbol of how humans build myth onto reality – to make the reality more bearable, or perhaps, more understandable. We took a common, brutal, petty serial killer and made a monstrous towering myth out of him that washed into, and coloured, our own werewolf folklore even until today.

Some early readers of my book have seen things the other way around, though – suggesting that perhaps I am using Stumpf to comment on the present day, masking a critique of the modern age with the myths of a time long gone. They see in the book – with its background of religious war and persecution, and the dreadful position that women were forced to hold in society – some sort of comment on what is happening in the 21st century. Some have mentioned Islamic State, and asked me if that is what the book is really about.

I just shrug and say, this story has been in my head since I was a wee boy with a high fever reading horror stories. I simply want to tell it. See in it what you want. As French thinker Roland Barthes said: "The author is dead." Once a book leaves me, it is up to you to see in it what you wish. You should imagine that, like a vampire or a werewolf, I do not exist at all.

Neil Mackay is the editor of the Sunday Herald, and also the author of The War On Truth, and All the Little Guns Went Bang Bang Bang.

His latest novel, The Wolf Trial, is available now, published in hardback by Freight Books, £13.99.

The public launch party for The Wolf Trial will be held this Wednesday, May 4 at Cup, Number 4 Virginia Court, in the Merchant City, from 6.30pm