Jemma Lilley, left, is on trial for the murder of Aaron Pajich, right. Photos / Supplied
The only motive for the murder of teenager Aaron Pajich by Jemma Lilley was because it was her "life's ambition" to kill someone by the time she was aged 25, a WA Supreme Court jury was told.
State prosecutor James McTaggart delivered his closing address at the start of the fifth week of the trial of Lilley, 26, and her housemate Trudi Lenon, 43, who are accused of murdering 18-year-old Pajich, who has been described as autistic but "high functioning".
"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," McTaggart said, referring to Lilley's writings and evidence given by her friend Angela McKibbin of her aim to kill someone before turning 25.
There was "no doubt" Pajich had been murdered with fatal knife wounds to his chest and neck.
There was also no doubt he was buried in the backyard of the pair's Perth home, a place where a security camera showed the two of them and Pajich on June 13, the last time he was seen.
"Between them, they did all that was necessary to cause Aaron Pajich's death," McTaggart said.
Once she had fulfilled that "bucket list" requirement to kill, she was so "full of herself and euphoric" that she couldn't help boasting about killing someone to colleague Matthew Stray.
She also couldn't help leaving incriminating messages to her "obsequious and sycophantic" fellow murderer Lenon later that night, talking about how excited she was experiencing things "she had not felt before".
Those two errors of judgment contributed to Lilley being charged with murder, he said.
There was evidence of the pair planning the murder, through phone messages discussing killing someone while referring to each other by their bizarre names SOS and Corvina, and buying cleaning products and concrete to cover it up, McTaggart said.
SOS was the name of a serial killer in a book Lilley had written while Corvina was the name Lenon used as a submissive participant in the bondage scene.
Earlier, the court heard from Kim Michelle Taylor. She had known Lilley very well for many years but stopped having anything to do with her about one month earlier.
They had previously exchanged many explicit messages about sado-masochism, with Taylor once allowing Lilley to cut her lightly with a decorative knife and to choke in their sex acts, she said.
Taylor also confirmed Lilley had been married to a now-deceased man named Gordon, who she referred to as 'Gacy' after the notorious American serial killer and rapist John Wayne Gacy.
"It was just a nickname," Taylor said.
She said the texts also heavily featured comments on the Dexter TV show, which is about a fictional serial killer.
The court also heard testimony from Lenon's 22-year-old son Jacob, who said his mother was interested in horror films.
He was asked if she liked horses, given a riding crop was found in her bedroom, but he said no.
It emerged on the first day of the trial, which is now into its third week, that Lenon was for many years a submissive participant in the bondage, discipline and sado-masochism scene, and the 'Corvina' character she assumed had been awakened by Lilley.
Lenon told detectives the pair didn't have a sexual relationship.
A text message from her to Lilley that was read out in a video-recorded police interview played on Wednesday read: "I am fortunate to have met my true dominant counterpart because without you, I would continue to feel shame.
" ... even writing this is highly erotic. You allow me to explore Corvina and I actually really like her.
"So for all of this I say thank you."
Lenon blames Lilley for the murder, but admits to being an accessory.
Lilley denies knowing how Pajich died and claims to have been asleep when he was last seen with Lenon.
The trial is due to finish this week.