Grandmother 'amped up' murder call: court

A drug dealer who told an associate her daughter's de-facto should be killed 'amped it up' by claiming he had physically abused her granddaughter, a Perth court has heard.

Travis Bennjamin Mills, 30, was on Good Friday last year beaten with a baseball bat in his home then taken in the boot of his car to a vacant lot, where it was set ablaze - burning him to death while he was alive.

His partner Seleena Monique Ruthsalz and her mother Joanne Christine Ruthsalz are among four people on trial in the Supreme Court of Western Australia charged with his murder.

Justine Claire Campbell - who bought methylamphetamine from Joanne, sold her cannabis and played a central role in arranging the murder - pleaded guilty before the trial commenced.

Her husband Darren Graeme Campbell and her old friend Darryl Raymond Newton are accused of carrying out the attack when Mr Mills was home alone.

Justine testified on Thursday, telling the court she and Joanne had previous violent relationships in common.

When Joanne told her Mr Mills was "a bit of an arsehole" and "was one of those", it reminded Justine of her fears for herself and her daughter many years ago, she said.

"It just pulled all my strings," Justine told the court.

"It triggered me in a million different ways.

"I could just see me and my daughter."

She said Joanne told her she didn't get to see Seleena and granddaughter Ruby much as Mr Mills didn't like her and her partner "because we're dirty drug addicts".

But they couldn't "do anything" about him because they were "too close", Justine claimed.

Seleena then told her she couldn't leave Mr Mills because he'd kill her or take Ruby away.

"She was terrified," Justine said.

"By this time, I've got it in my head 'he's one of those'.

"I feared for Seleena. I wanted to protect her."

Justine said she suggested to Joanne and her partner that they find a friend who could "bash" Mr Mills, but the grandmother said "for f*** sake, we need to kill the c***".

She claimed Joanne then told her Mr Mills had held his daughter under the shower until she gasped for air and had headbutted the 1-year-old, leaving a mark on her head.

"She was amping it up. Every time I went over there, she would say 'when is it going to happen?'," Justine said.

Seleena would also visit her hydroponics shop and "put pressure on".

When she told Newton about Ruby, he replied that Mr Mills "needs to be gone", but she claims she said he only needed a "touch-along".

Her husband, however, told her to "leave it alone".

The court was also shown video of Seleena's police interview, in which she said she thought Mr Mills would just have his legs broken.

"It just got out of hand and that's how it ended up in murder," she said.

"It was always going to be a possibility."

The trial continues.