We were odd children, yes, but why did we feel sorry for Jeffrey Dahmer?

As our childhood serial killer interest developed, Dahmer became a “favourite” for my brother and I.

When my brother and I were small, we lived in Muswell Hill, not far from Cranley Gardens, where Dennis Nilsen, who died last month at 72, dismembered his victims and flushed them down the drain. “Take us to look at Nilsen’s house!” we would whine to my dad, as we drove to Camden Lock on Saturday afternoons to help pack up my mum’s market stall. My dad would sigh and park outside the sad flat, which had lain empty for four years. We would look at the small dark windows on the top floor and wonder if there were still any bones in the drains. What ghoulish children. It recently occurred to me that we could only have known about the flat, and Nilsen, and his crimes, because my parents told us in the first place.

As our childhood serial killer interest developed, we got a “favourite”: Jeffrey Dahmer, the American Nilsen (same glasses, similar crimes) who looked a bit like David Beckham and – this made us very sad – was killed by a maniac in prison in 1994, three years after being sent down for his 17 abominable crimes. I feel bad saying “favourite” now. I distrust that tendency – with shows like Mindhunter, Dexter, Killing Eve – to find serial killers a little bit glam, though I’m guilty of it. But Dahmer’s central motivation – loneliness – did humanise him, as far as we were concerned: the fact that he killed his men so that they didn’t leave in the morning was easy for us children to grasp. Mind you, I’m still not sure how we could feel sorry for him. The policeman who finally got Dahmer’s confession liked him too – he lent him his stripy shirt for his court appearance.

Anyway. A weird new film – My Friend Dahmer – looks at Dahmer’s killer loneliness via the cliche of the indie high school movie: it is based on a graphic novel by his former classmate John “Derf” Backderf. Dahmer (played – this blows my mind – by the Disney star Ross Lynch) is, like many teenagers, lethargic, dead behind the eyes and of limited academic acumen. Unlike many teenagers, he likes to collect roadkill and dissolve it in acid given to him by his father, a chemist.

The film is predicated on one interesting and obscure piece of biography: that briefly, at Revere High School, northern Ohio, miserable, murderous Dahmer was a goof. A retiring, friendless teen, he began, one day, to act up, or “spazz out”, imitating a person with cerebral palsy, like President Trump did, which – this being the 1970s – made all the children laugh, who had until then completely ignored him.

The spazzing became known as “doing a Dahmer”. He did it in lunch queues and classrooms – then on request in libraries and shopping malls. Sixteen-year-old Backderf set up the Dahmer Fanclub. He drew pictures of his new friend as a superhero (and as a birdfeeder, and a flagpole, for some reason) – and for a while, if you believe the film, Jeffrey Dahmer had three friends: all of them boys, all of them desperate to get laid, all of them teetering on the edge of bad taste and violence. How easy it is to make a mark in high school if you try – though no one knows quite why he did it. He must have known he was not the same. While they were fantasising about each other’s moms, he was dreaming of sex with a glassy-eyed corpse, and you can’t explain that over a spliff in someone’s den. After a few months, he drifted away again (“he hasn’t spazzed in weeks”). For a prank, he’d snuck into every club photo in the High School year book. When it was printed, a teacher blacked out his face with a pen.

Dahmer killed his first victim just after graduation, in June 1978 – I’d never realised it happened so quickly. My Friend Dahmer is a strange film: it doesn’t glamorise, it doesn’t explain, and it’s really very boring in parts. But it explores the brief moment at which an increasingly disordered mind intersected with normality, tried it out, and left it again. Lionel Dahmer noticed that, at four, his son liked the sound that some dry animal bones made when he pulled them out from under the porch. He noticed – but he wasn’t worried. Why should he be? Our dad wasn’t worried when we nagged him to hang out near Dennis Nilsen’s drains.