A judge has said his hands are tied by inadequate legislation in sentencing the perpetrator of a "cruel and bloodthirsty action" who should be thought of as a threat to the community indefinitely.
Anthony Daniel Rajek was convicted of attempted murder over the stabbing of a 52-year-old man who was sleeping in the doorway of a city business.
Rajek came out of his apartment carrying a concealed knife and wearing a face mask about 3.20am on January 15, 2022.
Cameras filmed the then 21-year-old walk purposefully to the intersection of the Queen and Albert Street malls.
His victim - a man with complex mental health needs - lived in Coorparoo but slept in the particular city centre doorway as he believed people were out to kill him at night.
"He did that because in a grotesque irony he felt safe there," Brisbane Supreme Court Justice Peter Callaghan said when sentencing Rajek on Friday.
Rajek gave the man a brief glance, continued walking, then looped back in a "tactical manoeuvre" before plunging the knife into his victim's neck in a deliberate and calculating way.
He withdrew the weapon before walking steadily away, arriving home about eight minutes later.
Officers found blood on a hoodie, a washed knife and one empty cocktail glass in Rajek's barely furnished, tidy apartment about 10.30am.
"Nothing said by you or observed by police suggests that you were either intoxicated or delusional," Justice Callaghan told Rajek.
The injured man staggered to a nearby restaurant where he was given first aid that saved his life.
Justice Callaghan said it was a frightening prospect for Rajek to be in the community unsupervised without any explanation for the horrific offence even after psychiatric assessments.
Rajek reported hearing voices including threats from his silent victim, had suffered abuse and trauma as a child and had abused alcohol, cocaine and methamphetamines but was not diagnosed as a psychopath.
An appropriate sentence for the "cruel and bloodthirsty action" would put Rajek behind bars for a substantial time, longer if he did not take part in treatment and then be under supervision.
"Someone who has for no easily understood reason behaved in this way, even once, should be thought of as a threat to the community for the indefinite future," Justice Callaghan said.
But the law did not enable the judge to impose the sentence demanded in this case.
Sentencing legislation required him to consider the protection of people from harm and offenders' rehabilitation, he said.
But it also had the effect of "reducing these worthy aspirations to, what are in the circumstances of a case such as this, meaningless platitudes".
The serious violent offences scheme means Rajek is required to spend 80 per cent of his jail term behind bars with a limited incentive to engage in rehabilitation. The remainder would be in the community on parole.
Justice Callaghan said the Sentencing Advisory Council had recommended changes nearly two years ago that would ensure a serious offender such as Rajek could be supervised for longer.
"It just seems that this case raises squarely the inadequacy of the sentencing options that are available to me," Justice Callaghan said, telling the court his hands were tied.
He sentenced Rajek to 13 years and six months behind bars.
