Perth father 'burnt alive' in car boot

A 30-year-old badly beaten man was burnt alive in his car after his de facto wife and mother-in-law plotted his murder, a Perth court has heard.

The mother and daughter, Joanne Christine Ruthsalz and Seleena Monique Ruthsalz, are among four people on trial for the murder of Travis Bennjamin Mills, whose charred remains were found in the boot of his car on a vacant lot in Hilbert on Good Friday last year.

Three other people played a significant role in Mr Mills' murder, the Supreme Court heard during openings by prosecutor Carmel Barbagallo on Monday.

Central to the premeditated killing was a woman Joanne Ruthsalz sold drugs to, Justine Claire Campbell, who pleaded guilty before the lengthy trial could commence and will testify later, the court heard.

The other accused are Justine Campbell's husband and co-owner of their Bibra Lake hydroponics shop, Darren Graeme Kenneth Campbell, and her old school friend, Darryl Raymond Newton.

The court heard Seleena Ruthsalz and Mr Mills argued occasionally and had even discussed separating, but she and her mother manipulated the others into taking part in the murder plot by saying he beat her, Ms Barbagallo said.

The Ruthsalzs wanted to stay at arm's length from the murder to avoid detection by police, the prosecutor said.

Campbell did much of the organising but her husband and Newton executed the plan.

Mr Mills was first beaten with a baseball bat in his Seville Grove home then taken in the boot of his own car to the vacant lot, where it was set ablaze.

"Travis Mills was alive when the fire started," Ms Barbagallo told the court.

She said the motive for killing Mr Mills, a scaffolder, would perhaps never be known.

But it was clear his relationship with his mother-in-law was full of animosity, she said, while Seleena Ruthsalz was being manipulative when she told Justine Campbell - who had previously been in an abusive relationship - that she feared him.

Ms Barbagallo also told the jury that claims by Seleena Ruthsalz that Mr Mills planned to leave her and move to Queensland with their daughter Ruby - who turned one on the same day her father was killed - were nonsense.

While Mr Mills drank almost every night and smoked cannabis, he objected to his mother-in-law dealing methylamphetamine and heroin, and didn't want his daughter to visit the "filthy" house.

The court heard Mr Mills had been home alone building a trampoline, which was Ruby's birthday gift, on the night he was attacked.

Ms Barbagallo also told the jury that the Ruthsalzs cleaned up blood at the house in the early hours of April 3 last year.

The trial continues