Dahmer, dubbed The Milwaukee Cannibal, drugged, raped, murdered, dismembered and ate 17 men and boys from 1978 to 1991.
He strangled and stabbed them in a bloodfest that ended only after a would-be victim overpowered him and led cops to his fly-infested flat in Wisconsin.
But one of his old school pals insists he was “quiet and shy” at school and said he actually “liked him”, The Mirror reports.
John Backderf has spent years trying to come to terms with the horrific crimes committed by his childhood friend.
He eventually went on to write a book about his relationship with the serial killer, titled My Friend Dahmer.
The book has now been turned into a film by the same title, which hit cinemas yesterday.
And its author, now 58, insisted Dahmer was “not always a monster”.
He said: “The Jeff I knew had committed no crime.
“So yes, he was a monster – but not always. I liked him.”
Dahmer committed his first murder just three weeks after graduating at 18.
He lured 18-year-old hitchhiker Steven Mark Hicks to his house, before bludgeoning and strangling him.
He later dissolved the body in acid, crushing the bones and scattering them in nearby woodland.
This was the start of a killing spree so horrific it would catapult him to global notoriety.
When cops raided his flat 13 years later they found three human heads in his fridge, along with skulls, torsos, organs and hands scattered around his flat and more than 70 Polaroids of mutilated bodies.
But Backderf insists Dahmer was just “another sad kid with problems” and the horror could have been avoided if adults had reached out to the troubled teen.
He recounted how the young Dahmer was quiet when he first met him at the age of 12, but became more outgoing at high school.
He started disrupting class with bizarre pranks like pretending to have seizures – to the amusement of his group of pals, who would egg him on.
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Backderf recalled how Dahmer spoke fondly of those days from his prison cell as “the best time of his life” because he had friends.
He added: “I don’t think it’s any coincidence he began to kill when school ended.”
He told how he felt “numb” years later when he heard the horrific details of his former pal’s crimes.
Backderf said: “With the snap of a finger my entire personal history changed.
“All these silly things we had done in high school, just like that they became very sinister.”
But he insists he never felt sympathy for Dahmer, who was battered to death in prison by another inmate in 1994, saying: “He made the choice to kill.”
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