Aaron Pajich's only mistake was to cross paths with someone whose life ambition was a desire to kill.

The Perth teenager met Trudi Lenon, 43, through her teenage son, who he was friends with, and he studied at the same TAFE as her. Lenon and her children lived with Jemma Lilley, a 26-year-old who had been obsessed with murder for much of her life.

Mr Pajich, 18, who had Aspergers, was trusting and kind. The Western Australia Supreme Court heard he lived in a childlike world.

His cruel killers lived in a fantasy world too - but their world was anything but childlike and more the stuff of nightmares.

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Lilley had harboured thoughts of being a serial killer since she was a teenager growing up in England, and later boasted about the murder to a supermarket colleague.

Both she and Lenon were found guilty yesterday by a jury who deliberated for less than three hours. They'd sat through five weeks of evidence that was so gruelling the judge offered them free counselling once the verdicts were delivered.

Lilley finally got her wish in June last year. At trial Lilley tried to blame Lenon, admitting to being an accessory, but Lenon didn't give evidence. She denied knowing how Pajich died and claimed to have been asleep when he was last seen with Lenon.

The jury rejected both their accounts and found them equally to blame for firstly planning the murder, then killing Mr Pajich, and finally trying to cover up the crime.

Their case and their sick obsessions provide a rare glimpse of female thrill-killers.

Sick and twisted fantasy

They were quite a team. Lenon was active in the local BDSM scene and was known to Lilley as 'Corvina', a name she assumed in her bondage and submissive sex sessions.

Lilley was anything but submissive. She was fixated by sharp knives and her pet name from Lenon was SOS, the same name she'd given a character in a dark book she wrote as a teenager.

"At the time of the murder, she was a person obsessed with violence and all kinds of unquestionably cruel manifestations of torture and was writing about it," Crown prosecutor James McTaggart said in his closing address.

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She'd told a friend and even wrote about her growing desires to take a life, something she wanted to achieve before her 25th birthday.

The pair brought cleaning products and concrete in the lead-up to the murder during trips to Bunnings. The trips were also used to buy 100 litres of hydrochloric acid which they likely planned to use to dispose of Mr Pajich's body.

Prosecutors believed Lilley selected Mr Pajich because he would be an easy target. He was slight, weighing only 51kg, while Lenon was obese. He was vulnerable and trusting and spent much time walking alone, so if he didn't return home quickly they figured they get away with murder.

They might have done that if they'd kept quiet.

The twisted writing of Lilley gave investigators plenty of clues. "I feel that I can't rest until the blood of a fresh, screaming, bleeding victim is gushing out and pooling on the floor," one passage read.

The texts messages between the pair did too.

"The first time it will be very controlled ... brutal and SOS will be totally entranced by it," Lenon wrote, adding it was "definitely" time. "I am ready."

On June 13 last year - on the night before Mr Pajich was murdered - Lilley texted Lenon that she was feeling "things I haven't felt before. It's incredibly empowering".

Pajich, 18, who was found buried underneath a concrete slab. Photo / Facebook
Pajich, 18, who was found buried underneath a concrete slab. Photo / Facebook

The murder

The following day they laid the trap for Mr Pajich. The young man was lured to the suburban house Lilley and Lenon shared on the pretence of borrowing some new computer games from Lenon's son.

The trial heard Lilley attacked him from behind soon after her arrived. She overpowered him and tried to strangle him with a garrotte before Lenon joined the fight. She held him down while Lilley took to him with a knife stabbing him to death.

Once he was dead they wrapped his body in a locked room. He was later buried in their backyard.

In some of her writing, Lilley had told of wanting to hear a victim scream and fantasised about pools of blood. Given the trace evidence that was left in the home she got what she wished for.

When they scoured the home for clues, homicide detectives found DNA traces and blood splatters throughout the home. Outside the burial site was just as crude.

The grave was covered with a tarpaulin, the concrete had just been poured, and tiles were laid sloppily.

Mr McTaggart told the jury the scene was "ridiculous". And so was what happened next.

The confession

In keeping with her killer ambitions, Lilley wanted it known she had killed. In a move that not only led to her conviction but that of Lenon, she confessed to a colleague. She would later say it was just fantasy talk, but the boasts were full of so much chilling detail, police realised it was the truth.

Matthew Stray stacked shelves at the Woolworths where Lilley worked. Mr Pajich was still officially listed as a missing person when she told him that killing him was "harder than I first thought" and that "Trudi had to hold him down".

Mr Mactaggart said she "could hardly wait to blurt out that she had stabbed a person and finally killed somebody" but assured her colleague "don't worry [the next killings] will all be vigilante".

The following day Lilley tried to assure Mr Stray she was joking. In response to a text from him asking "did you do it" she started to backtrack by claiming 'it's all an act, in character. Nothing's happened."

Mr Stray didn't believe her. He went to police and very soon the sick and twisted world of Lilley and Lenon came crashing down.

When they learned of the jury decision yesterday they both stayed silent. But outside court, Mr Pajich's mother Sharon Pajich unleashed on the evil pair.

"They are disgusting animals ... they can rot for all I care. (They should get) a lifetime, no parole. They don't deserve the air they breathe in."

She called for them to never be released.

"You can't get it out of your head, and it will be a lifetime for me. They deserve everything they get for what they've done."