A Perth woman fatally stabbed by her ex-husband had thrown herself on her pregnant daughter to protect her from the knife attack, the Supreme Court has heard.

James Payet attacked Selina Bello in a Como hairdressing salon before turning the knife on her mother Angela Ferullo, 43, in June last year.

A customer who tried to intervene was also wounded.

Payet, 48, pleaded guilty to murdering Ms Ferullo and causing grievous bodily harm to Ms Bello, who was 22 weeks pregnant at the time.

Sentencing submissions were heard today.

The Supreme Court heard Payet made repeated threats to Ms Ferullo and her family in the months before the attack.

Prosecutors told the court Payet and Ms Ferullo separated after a 12-year relationship in 2011, but he continued to live with her at her home.

In the months before the murder their relationship deteriorated and on one occasion he held a knife at Ms Ferullo's throat and said he would slit it if she went to the police or sought a restraining order.

Prosecutor Linda Petrusa said in the days before her death, Ms Ferullo and her daughters received "a series of abusive and derogatory text messages" including one saying he was "ready to decimate anyone...who crosses my path".

Victim sought restraining order on day of attack

The court heard Ms Ferullo did go to police the day before her death and obtained a 72-hour order banning him from her home and from contacting her.

Ms Ferullo also planned to go to court to get a restraining order on the day of her death.

She was on the telephone to police when he arrived at her daughter's hair salon, the court heard.

Prior to arriving there, Payet let himself into Ms Ferullo's home in the suburb of Success and hid in the roof space before holding her lodger hostage and threatening him.

James Payet pleaded guilty to murdering his former wife Angela Ferullo. ( Supplied. WA Police )

The court also heard Payet initially attacked Ms Ferullo's daughter, who was 22 weeks pregnant.

Ms Ferullo threw herself on her daughter to protect her, and Payet dragged her off and repeatedly stabbed her.

A 67-year-old customer threw a chair at Payet during the attack to try to stop the attack.

Payet fled the scene, prompting a massive manhunt but was arrested later in the day when he presented at a hospital in Murdoch with self-inflicted knife wounds.

His lawyer Laurie Levy SC said his client's actions were "an almost inevitable consequence of his background, mental health problems and ...drug-taking".

Payet faces the prospect of life in jail when he is sentenced on Friday.

Ms Bello gave birth to a healthy baby boy late last year.