A man found wandering naked on a highway in WA's South West two years ago had earlier choked to death a mother of three because he believed she was a member of the Coffin Cheaters bikie gang, the Supreme Court has been told.

Key points: Ian Dennis Jones admits he strangled Tanya Beattie at his Bunbury home in late 2016

Ian Dennis Jones admits he strangled Tanya Beattie at his Bunbury home in late 2016 The court heard he has paranoid schizophrenia and a long history of psychiatric illness

The court heard he has paranoid schizophrenia and a long history of psychiatric illness He tried to get admitted to hospital and previously tried to choke his sister

Ian Dennis Jones, 52, has admitted killing 41-year-old Tanya Beattie at his house in the Bunbury suburb of Eaton in November 2016, but has maintained he was not guilty of her murder because he was insane at the time.

At the beginning of Mr Jones's trial, state prosecutor Brett Tooker said Ms Beattie was a "chronic alcoholic" who had become "drinking buddies" with Mr Jones after regularly walking past his house on her way to a friend's place.

Mr Tooker said Ms Beattie was last seen by members of her family when she left their house of the afternoon of November 4.

He said later that evening Ms Beattie and Mr Jones were seen arguing at his home before one of Mr Jones's neighbours reported hearing a woman screaming about 11:00pm.

Ms Beattie's body was found at Mr Jones's home in Bunbury in November 2016. ( ABC News: Roxanne Taylor )

Ms Beattie's body was found the next day by Mr Jones's mother, who had gone to the property after police contacted her saying her son had been arrested while wandering naked on the South West Highway near Boyanup.

The court heard Mr Jones had also been seen earlier at an industrial estate, "splashing in mud" and "talking nonsensically".

Mr Tooker said it was the state's case that sometime between 11:00pm on November 4 and 7:00am on November 5, Mr Jones had choked Ms Beattie because "he thought she was a Coffin Cheater and he blamed them for the death of his son in 2002".

Killer tried to get admitted to hospital

The court heard Mr Jones had a long history of psychiatric illness and in 2013 had been diagnosed with paranoid schizophrenia.

Mr Tooker said about a month before Ms Beattie's death, Mr Jones had gone to the emergency department of Bunbury Hospital and said he was "feeling unwell mentally" and "wanted to get on top of things".

Mr Jones previously went to Bunbury Hospital complaining of mental health problems. ( ABC News: Gian De Poloni )

The court heard Mr Jones was discharged the next day after being assessed as "not voicing any suicidal or homicidal ideas", but Mr Hooker said his behaviour during the next few weeks "could only be described as bizarre".

In statements read to the court, members of Mr Jones's family described incidents in which he destroyed property and plants and described himself as God.

The court also heard that in June 2016, five months before Ms Beattie's death, he was admitted to hospital after attempting to choke his sister.

Mr Tooker said a psychiatrist, who had examined Mr Jones, had concluded that at the time of Ms Beattie's death he was in "a psychotic state" and "not capable of controlling his actions".