One of WA's most notorious murders is back on the unsolved list after a decision to drop charges against a Perth man over the death of alleged child sex offender Robert Dalliston almost a decade ago.

Mr Dalliston, a 69-year-old man who lived in Mandurah, was found dead in his home on January 15, 2009.

It is believed he was killed while he was asleep, and police at the time said it was not a random attack.

Robert Dalliston was killed in his Mandurah home in January, 2009. ( WA Police )

They said the level of violence suggested the murder was committed by someone who held a grudge.

Mr Dalliston was facing child sex charges and, although the exact cause of his death was not revealed, the crime was described as "particularly violent" and "brutal".

"No matter what a person has done in their life, no one ever deserves to die like this," then Detective Senior Sergeant Kevin Bradshaw said at the time.

There was an extensive police investigation that included numerous public appeals and the release of CCTV footage of Mr Dalliston paying a bill at the Mandurah post office seven days before his body was found.

However it was not until five years later that Ernest James Mitchell was charged with his murder.

Five year fight against charges

Mr Mitchell denied the allegation, and he and his lawyers have spent the past five years arguing over the evidence that should be allowed at his trial, which was set down for five weeks starting in October.

A large part of case against him was based on DNA evidence taken from the crime scene, which was examined by various overseas experts who testified at pre-trial hearings.

Ernest James Mitchell was accused of killing alleged child sex offender Robert Dalliston in Mandurah, but the charges were dropped. ( ABC News )



Mr Mitchell's lawyer, Jonathan Davies, had argued the levels of DNA were so low they were unreliable and should not be admitted as evidence, but a judge ruled the evidence should be allowed.

On Friday state prosecutor Justin Whalley told the Supreme Court that after consultation with key witnesses, a re-evaluation of the case had led to a conclusion that there were no reasonable prospects of a conviction and the charge against Mr Mitchell should be discontinued.

He had been granted bail after spending about two years in jail awaiting trial, but he was not in court for Friday's hearing.

Outside the court, Mr Davies said his client was "relieved to be vindicated".