One of two women accused of murdering a Perth teenager who had autism said she wanted to kill someone before she turned 25, a witness has told WA's Supreme Court.
Jemma Lilley is on trial along with Trudi Lenon for the murder of 18-year-old Aaron Pajich — whose body was found under red tiles and a slab of concrete in June last year in the backyard of the Orelia house the women shared.
Mr Pajich had been stabbed in the chest and neck and also had an injury consistent with being garrotted.
It is alleged he was lured to the property to be murdered.
The court heard Ms Lilley — who was 25 at the time of the alleged murder but has since turned 26 — was a regular customer at a video store in Girrawheen managed by Angela McKibbin.
Ms McKibbin testified that around mid-2013, the pair had a conversation in which Ms Lilley told her "she wanted to kill someone and she wanted to do it before she was 25".
Ms Lilley told her the "feeling" she had "was getting stronger", Ms McKibbin said.
Ms McKibbin told the court when she replied by saying "it's pretty hard to get rid of or hide a body", the accused replied "there were a number of ways of doing it".
Ms McKibbin said she did not tell anyone about the conversation until after seeing the case in the news last year, when she rang Crime Stoppers.
Video store account named after 'Son of Sam'
Under questioning from Ms Lenon's lawyer, Ms McKibbin said Ms Lilley mostly used to rent DVDs with titles about horror and serial killing.
She said her video store account was in the name SOS, which stood for Son of Sam.
The court has previously heard SOS was tattooed on Ms Lilley's arm and was the name of the main character in a fictional book she had written about a serial killer, called Play Zone.
Under cross examination, Ms McKibbin rejected the suggestion Ms Lilley had said she was going to be killed by 25, and not that she was going to kill someone.
Both women deny murdering Mr Pajich, who had been assessed as being on the autism spectrum.
Ms Lilley has denied any involvement in his death, while Ms Lenon has said she only helped clean up the scene after Mr Pajich was killed by her co-accused.
Police found 'concealed' room in house
Later, a police officer revealed details of what was described by prosecutors as a "concealed" and "sealed" room in the Orelia house.
It was discovered on the third day of police forensic examinations of the property and Senior Constable Derek Pitcher said officers had to "sledgehammer" off the lock of the door because it appeared to be fitted "back to front".
Photographs of the room tendered to the court showed three of the room's walls were covered in bright blue tarpaulins, with two windows underneath them also covered in black plastic, so no one could see in or out of the room.
Constable Pitcher said there was a red tool box in one corner and what appeared to be a shopping trolley cut down to its base, with two wooden boards secured on top.
He testified he noticed there was a clump of hair around one of the trolley's wheels and there was also a single strand caught in the frame, which he said all appeared to be human hair and dark in colour.
Constable Pitcher said the boards on the trolley had some red staining, which could have been blood, while a cloth found near a mop appeared to have blood on it.
However, he said an examination of the tiled floor did not find any evidence of what was called "a bloodshed event" occurring in the room.
The court on Wednesday heard how a collection of knives, a garrotte and a notebook containing a list of torture methods were among the dozens of items seized from the women's house.
CCTV footage of the moment Mr Pajich followed the pair into the house has also been played to the court.
Mr Pajich's body was found wrapped in a tarpaulin and his face was covered in cling film in what prosecutors have said was an "amateurish attempt" to conceal the crime.