A 43-year-old woman lured a teenager with autism to a suburban Perth house so her 26-year-old friend, who had written a book about a serial killer, could feel the "euphoria and exhilaration" of murdering someone, WA's Supreme Court has been told.

The allegation was made at the start of the trial of Trudi Lenon and Jemma Lilley, who are accused of murdering 18-year-old Aaron Pajich in June last year.

The teenager's body was found in the backyard of Lilley's Orelia home, wrapped in a tarpaulin and buried under tiles and a slab of concrete, in what was described by prosecutor James Mactaggart as an "amateurish attempt" to conceal the crime.

His face was covered in cling film.

In an opening address to the court, Mr Mactaggart said the teenager studied at a local college with Lenon and was also a friend of one of her three children.

He alleged the two "brutally and violently" murdered Mr Pajich after the friendship between the two of them took on "sinister tones".

He said days after the alleged killing, Lilley divulged what had happened to a work colleague at the Palmyra supermarket where she was a supervisor, telling him "I did it, didn't I".

"It was harder than I thought it would be … there was blood everywhere," she is alleged to have said.

Mr Mactaggart said Lilley later tried to downplay what she had said by sending text messages to the colleague describing herself as a good story teller and claiming it was "all an act".

Mr Pajich was buried under a slab of concrete in the backyard of an Orelia house. ( ABC News: Irena Ceranic )

However, Mr Mactaggart said Lilley had divulged to the colleague that Mr Pajich had been stabbed — a detail that at the time was not known to anyone including police.

Mr Mactaggart also alleged the day after the murder, Lilley sent a text message to Lenon saying: "I'm seeing things I haven't seen before, I'm feeling things I haven't felt before … it's incredibly empowering."

He said evidence to be called at the trial would include other statements she had made to people, details of a book about a serial killer she had written in 2007 called Play Zone, and a plan for her to get tattoo of a jester holding a knife.

"The reason why Lilley murdered someone who was a complete stranger to her … is that she wanted to experience what it was like to kill someone, to take somebody's life," Mr Mactaggart said.

"She murdered Aaron Pajich for the euphoria and exhilaration of it."

Mr Mactaggart said it was alleged that Lenon, "out of some warped or misguided reason" lured Mr Pajich to the house and "facilitated" or "aided" the murder.

"She lured Aaron Pajich to his certain murder," he said.

Both women deny murdering teen

The court heard it could not be proven who actually inflicted the stab wounds, but it was the state's case both women were criminally responsible.

"On the state's case the two of them acted in concert to murder him," he said.

Both women deny murdering Mr Pajich.

Lilley's lawyer John Prior said his client maintained she was not involved in murdering Mr Pajich, cleaning up the scene or burying his body.

Lenon's lawyer, Helen Prince, said her client denied luring the teenager to the house, but admitted she was present when he was killed by Lilley.

However, Ms Prince said Lenon had no idea Mr Pajich was going to be murdered, and although Lilley had previously expressed a desire to kill a person, she did not take her seriously.

"She [Lenon] has been involved in BDSM, involved in world fantasy sex play, as a submissive," she said.

"Because of her previous engagement in role play, that's how she saw it, from her point of view this was fantasy. It wasn't real."

Ms Prince said Lenon admitted cleaning up the scene and lying to the police, but she maintained she did not know Mr Pajich was going to be murdered.

The trial is set down for five weeks.