COPENHAGEN (AFP) - Danish submarine builder Peter Madsen was well-known at home for his eccentric ambitions to develop rockets and space travel, but few knew about his macabre fantasies involving violent sex, beheaded women and snuff films.

On Wednesday (April 25), the 47-year-old self-taught engineer was sentenced to life in prison after a Copenhagen court found him guilty of the premeditated murder and sexual assault of Swedish journalist Kim Wall onboard his home-made submarine last year.

Nicknamed "Rocket Madsen", he has described himself to friends as a "psychopath, but a loving one".

But a court-ordered psychiatric evaluation found him to be "dangerous", a "perverse polymorph" with "psychopathic traits" and at "high risk" of recidivism.

Madsen insisted Wall's death was accidental, but he confessed to chopping up her body and stuffing her head, arms and legs into plastic bags, weighing them down with metal pipes before tossing them into the sea.

The high-profile trial unveiled his interest in violent sex and snuff films of women being beheaded, skinned, tortured and impaled.

Prosecutors said Madsen tied Wall up before beating her and stabbing her repeatedly in the genital area and that he planned it all along.

But Madsen said Wall died when the air pressure suddenly dropped and toxic fumes filled the vessel while he was up on deck.

ROCKY PAST

Peter Langkjaer Madsen grew up in the small town of Saeby, 100km west of Copenhagen.

His parents divorced when he was six and Madsen went to live with his father, whom he has described as authoritarian and violent.

"When I think about my father, I think how children in Germany must have felt if their dad was a commandant in a concentration camp," Madsen said in a 2014 biography.

At 15, he started his first company, Danish Space Academy, to buy spare parts to build a rocket.

He studied engineering, but quit once he thought he knew enough to build submarines and rockets.

"My passion is finding ways to travel to worlds beyond the well-known," Madsen wrote on the website of his now-defunct Rocket Madsen Space Lab.

In 2008, he launched the Nautilus, the biggest privately made submarine whose ownership was later transferred to him after a row with former colleagues.

Around the same time, he developed his idea for private space travel and in June 2011 successfully launched a rocket from a floating platform on the Baltic Sea island of Bornholm.

Madsen reportedly had an open marriage. Some of his ex-girlfriends have told the media he was into sado-masochism and erotic asphyxiation.

'UNTRUSTWORTHY'

His half-brother Benny Langkjaer Egeso told AFP after his arrest that Madsen is "very strange", but also "very open and likeable".

But others describe him as an erratic person who had spats with former colleagues and an interest in violent pornography.

"His sexual fantasies slowly got out of hand," an associate, who had worked in Madsen's laboratory, told the Copenhagen court.

The associate said Madsen toyed with the idea of making a pornographic film showing acts of torture and was "interested in snuff films," movies where a person is really killed or kills themself.

Madsen denied searching for or downloading such films but admitted to watching them "to be able to feel emotions and to cry" about the women's suffering.

The court found Madsen's explanations "untrustworthy" after he changed his version of events several times, and provided no reasonable explanations for why he brought a saw, plastic strips and a sharpened screwdriver on board just before the voyage with Wall, nor why he dismembered her or threw her overboard.

"He committed a cynical, planned murder, of a particularly brutal nature," the judge said.