Murder is a notoriously difficult charge for police and prosecutors to get to stick in the ACT. Unlike all other Australian jurisdictions, here it is not enough to kill someone in the course of committing an offence, you have to ''intend'' to kill. That's the main reason why no one has been convicted of murder in Canberra for more than 10 years.

But with Glen Porritt's case, police and the prosecution had what seemed one of their best chances of breaking the drought. As police saw it, all the elements associated with murder were there: motive, premeditation, and behaviour associated with consciousness of guilt. Then there were the online chat-room discussions with his sisters eight months earlier, in which he said that his parents ''deserve to die'' and ''if it were not illegal I would stab them''.

Add to that what appeared to be a frenzied stabbing attack, a clumsy attempt to make it look like a burglary gone wrong (items were taken from the scene), and a suspect who could not be found, and a seemingly solid case began to build.

But the test for the prosecution is not to show that its version of events is more believable, but to show that it is true ''beyond all reasonable doubt''. When presented with a plausible, albeit unlikely alternative version, many a judge and jury has acquitted.

This, it seems, was the difference between Porritt facing a life sentence for murder and a significantly less penalty for manslaughter.

The judge-alone trial before Chief Justice Terence Higgins opened with William Dawe, QC, saying it would be the Crown's case that Porritt had, through his sister Amy, ascertained their mother would be home alone that night. He attended the house earlier that day on the pretext of returning a book to Amy, and then waited for his mother to return from work.

Armed with a kitchen knife, he stabbed and slashed her in the face, neck, chest, arms and back, inflicting 57 incised wounds. This was cold-blooded murder.

Porritt's version? He had gone back to the house on a whim, having returned to the suburb to go to a video store. After a year of not talking to his parents, he chose that day to demand they give him money to compensate him for his childhood suffering.

When Mrs Porritt asked him to leave the house, he refused. She grabbed a knife and ran upstairs to her bedroom to call police. He followed. She lunged at him with the knife, he wrestled it out of her hand. She blocked his path to the door. He told the trial, ''I panicked and I was just trying to frantically get out of the room and started flailing my fists at her, trying to sort of hit her and shove her out of the way.''

The only problem was that he was holding a knife.

He fled and walked to the Acton shores of Lake Burley Griffin, where he disposed of the stolen items and the knife and fell asleep for four or five days.

Did anyone see him lying there for days? We don't know. He wasn't pressed on details during his cross-examination.

Chief Justice Higgins said yesterday, ''I found the offender to be a truthful witness and I accept fully his account of how the death of his mother occurred''. Case closed.