When Stuart MacBride is making up a serial killer, there is one paramount rule he follows. “It doesn’t have to make sense to us, as long it makes sense to the killer.”

The bestselling Scottish novelist has studied all sorts of books about what motivates killers – the FBI’s crime classifications handbook; FBI agent Robert Ressler’s books on tracking down serial killers – and that is his abiding takeaway.

“There was one guy, Richard Chase – if your front door was locked, he went away because he clearly wasn’t welcome. But if it was unlocked, that was an invitation to kill everyone inside,” MacBride says. “He thought he was being poisoned by Nazi UFOs via his soap dish, and the only way to stop his blood turning into powder was to consume his victims’ blood and internal organs. The act has to make perfect sense to the person who’s doing it. That’s what I have taken from the research. So when I’m writing I think, if I was that person, what would make sense?”

MacBride is speaking from his home in the wilds of north-east Scotland, where he lives with his wife and a multitude of cats (Grendel, his Maine Coon, was bought with the proceeds of his very first advance, some 16 years ago). His new novel, The Coffinmaker’s Garden, opens in barnstorming fashion as serial killer Gordon Smith’s home crumbles into the sea, revealing the human remains he hid in his garden. Ex-Detective Inspector Ash Henderson, who MacBride has put through the mill in his previous appearances (in Birthdays for the Dead, Ash’s daughter is the victim of a serial killer abducting and torturing girls), is put on the case.

Like all MacBride’s thrillers, it is a brutal, visceral read, laced with the blackest of humour and travelling to some very dark places. Reading it as more tiers of restrictions were imposed on the UK and a lonely Christmas loomed, I found it weirdly comforting to be drawn into a world of horror and murder and mayhem. MacBride has a good idea about why this might be.

“We’ve always loved these kind of stories. You go right back to the days when we lived in caves, and we would sit around our fires and tell stories of the monsters that were outside in the darkness. Crime fiction does exactly that thing,” he says. “The monsters we are scared of as a society are the ones that we see in crime fiction, and the detectives are superheroes without capes. They are King Arthur. They are Beowulf. These are the people that we have always gravitated towards, in our fictions and our stories.”

MacBride has loved crime fiction ever since mainlining the Hardy Boys as a child. He tried writing his first book in his mid-20s, as a couple of friends were giving it a go; he wrote a “dreadful” comedy crime novel, but persevered and landed an agent. His fifth attempt at a novel, Cold Granite, became his debut: the first in the Logan McRae series, it sees the Aberdeen detective sergeant out to catch a child killer who is stalking the city’s streets. MacBride called up the Grampian police and a hospital mortuary, asking questions about everything from police procedure to rigor mortis.

He had studied architecture at university before giving that up, and was working as a project manager for an IT conglomerate when he learned he would be published: “I was sat in this room in Guildford with all these coders, going through organisational this and testing strategy that, then the email comes saying I had a three-book deal.” He told his wife but no one else, keeping the deal secret for more than a year until the book was published in 2005. After an 18-month sabbatical, he decided to become a full-time writer; his last book, All That’s Dead (2019), was his 12th novel following Logan McRae. The Coffinmaker’s Garden is the third in the Ash Henderson stories – all packed, as he writes on his website, with “horrific crimes, murders, serial killers and much eating of chips and drinking of beer”.

“I do love a serial killer and I do love writing about fictional serial killers,” he says, “but I cannot stand reading about real ones because I can’t get past the fact these are real people, and the people they kill never get remembered – it’s always the person who does the killing. I love writing and reading about made-up ones because no one was hurt in the production of a book.”

What’s it like, writing such disturbing, visceral scenes? “I never sit down and think, ‘What would be the goriest thing I could do?’” MacBride says. “It is hard to write, but not because I’m thinking, ‘Oh dear lord.’ It’s more about how I get you to feel that, rather than me. How do I get that reaction from the reader? I’m coming at it as a challenge – how do I get you to feel this is something horrible?” He’s only made himself gag once, when writing a scene from the perspective of a man who had to eat a human eye or die. Describing it to me on Zoom – texture, crunch, saltiness – almost makes me gag as well, so I can see why; the scene was included in the second book he wrote, which has never been published.

The only other time MacBride admits to being frightened was when he was calling in one of his cats, Beetroot, from the patch of woods behind his house, and saw two eyes gleaming back at him in the torchlight. It wasn’t Beetroot, a rescue cat with only one functioning eye.

“Everybody else was in the house,” he says. “So I said to these eyes, ‘what are you, a fox or a badger?’ And this little voice in the back of my head said, ‘I’m something much much worse.’ I gave myself the utter willies with that, locked all the doors, thank you very much.”

This became the start of All That’s Dead. “A pair of eyes glittered back at him – too far away to make out anything but their reflected glow,” he writes in that novel. “There’s a papery rustling sound. Then a cold metallic one as a ghost-white arm appears from behind Nicholas, painfully bright in the torch’s glow. The arm holds an axe, the blade chipped and brown with rust. ‘A fox or a badger?’ A small laugh. ‘Oh, I’m something much, much worse …’”

• The Coffinmaker’s Garden is published on 7 January. To order a copy, go to guardianbookshop.com.