An associate of a white supremacist group who confessed to police he had taken part in murdering a man said he admitted the crime because his grandmother told him to tell the truth, a Perth court has heard.

The associate, who cannot be identified, was giving evidence at the Supreme Court trial of three other members of the right-wing nationalist group, Aryan Nations, who are accused of murdering Alan Taylor, 42, as he lay in bed at his Girrawheen home in April 2016.

It is alleged Mr Taylor's partner and the mother of his young son, Melony Attwood, 37, planned the murder with her lover, Aryan Nations President, Robert Edhouse, 22, so she could get access to Mr Taylor's life insurance.

The Sergeant at Arms of the group, Corey Dymock, 21, and the unidentified associate were allegedly "roped in" to help them.

State prosecutor Justin Whalley had previously described the group as a "death squad", telling the court Mr Taylor was hit with a hammer by Mr Edhouse, Mr Dymock and the associate, while Ms Attwood turned up music to "drown out the sound" of what was happening "before going around the house" to disturb items to make it look like there had been a burglary.

Mr Whalley said the associate later "broke ranks and decided to come clean" to police about his involvement because "he just couldn't live with the guilt".

Associate lured by promise of $100,000

Alan Taylor was found dead at his Girrawheen home in April 2016. ( Supplied: WA Police )

The court has heard he pleaded guilty to a murder charge, and was given a reduced sentence in return for testifying at the trial of the other three, who have pleaded not guilty.

In his evidence, the associate said he first became involved in the alleged murder plot because he was promised he would be paid $100,000.

He admitted initially lying to police by denying he had anything to do with the murder, but under cross-examination from Mr Dymock's lawyer, Justine Fisher, he said he decided to admit what happened after speaking to his grandmother.

"She knows when I lie. She can tell when I lie," the associate said.

"She said us grandparents aren't well and if you don't tell the truth, you'll get life and you won't be able to see us."

Ms Fisher also questioned him about other differences in the various version of events he had given to police, but he told the court he had no explanation for them and did not know why he had said different things.

The trial is expected to run for about another four weeks.