Unsure why he slashed the necks of an 82-year-old woman and her son while buying a printer, Adam Curtis Brown thought paint fumes may be to blame.
Brown, 43, was affected by pills and alcohol when he went berserk at Hervey Bay north of Brisbane in April 2018, the Brisbane Supreme Court has heard.
But Brown still can't explain why he launched a sustained, senseless, unprovoked attack on a 54-year-old man and his elderly mother with a retractable blade.
Brown arrived at their property to discuss buying the printer but became agitated and erratic, at one stage removing his shirt.
Told to calm down, Brown stabbed the man in the neck with a box cutter in his possession.
The man thought he was going to die and called out to his mother.
"He got his mother's attention so she could call an ambulance but as he was trying to yell blood was spurting out," crown prosecutor Chris Cook said on Friday.
Overall the man lost half a litre of blood and suffered a severed windpipe, later requiring a tracheotomy.
He was still able to tackle Brown after he assaulted his mother, cutting her hand.
It couldn't stop Brown from again attacking the elderly woman who was visiting from the UK.
Ignoring her pleas to stop, Brown slashed her neck before turning his attention again to her son.
Brown pursued the man as he tried to get help, throwing a metal fence post at him before driving away.
Brown was picked up 10km away by police after exhibiting "bizarre behaviour".
He was abusing his medication and drinking wine before the attack, but defence barrister James Godbolt said Brown was genuinely at a loss to explain his bizarre conduct.
His matter was delayed as Brown - who has been in custody for four-and-a-half years - explored the possibility that he may have been affected by paint fumes during the attack, the court heard.
But the Crown obtained a forensic medical officer's report that ruled it could not have contributed in any way.
Mr Cook said Brown was remorseful now his "intoxicated by paint fumes" theory had come to nothing.
Justice Peter Davis interjected: "But the point about that is he might be questioning himself, you can't say that (remorse) is not genuine.
"I mean he turns up to buy a ... printer and ends up going berserk. He is probably a bit confused about the whole thing himself."
Brown - who appeared via video link - pleaded guilty to four charges including grievous bodily harm with intent.
He was sentenced to nine years in jail with a serious violent offender declaration so must serve 80 per cent of the sentence before eligible for parole.
