Backman - who admitted the manslaughter of Les Brooks, shooting another man during a home invasion and seriously assaulting corrective services officers - has been behind bars since his surrender in 2016.

But the now 31-year-old says he stopped using drugs in the last two years once isolating from the general prison population.

Backman was carrying a sawn-off shotgun when he went into the Sunnybank Hills caravan park home of Wayne Francis Dean looking for drugs in October 2016, a Brisbane court was told on Friday.

Mr Dean survived but sustained a three-centimetre gunshot wound in his chest.

On November 17, 2016 Backman shot Mr Brooks, after having been in a relationship with that man's ex-partner for a few months.

Backman believed Mr Brooks had been violent towards the woman during their nine-year relationship although police knew of only a few alleged incidents, prosecutor Mark Green said.

About a week after Mr Brooks found Backman and the woman in bed together, he barged into the house in a rage.

Mr Brooks agreed at the insistence of Backman - who had armed himself with a shotgun he had kept hidden high in a closet - to go into the backyard.

Backman was backed up to the fence being goaded by Mr Brooks who was close enough to be able to grab the gun when he fired the fatal shot about 4pm.

The shooter messaged the woman saying he never meant it to happen.

"But I do know that I can sleep OK knowing that you'll always be safe now and that them two beautiful kids will never have to witness any more of his crap," he wrote in the message Mr Green read to the Brisbane District Court.

Backman called triple zero at 11.29pm from outside Brisbane's Roma Street watch-house saying he shot someone that day and wanted to hand himself in.

"He said he had the firearm that was used and that it was down his pants," Mr Green said.

Officers arrested him there with a shortened double-barrel shotgun loaded with one live round.

The Crown said Backman was to be sentenced on the basis he used the gun in self-defence, but with excessive force.

Mr Brooks' mother Caroline Krezic sobbed while telling the court of her constant battles and many meltdowns.

"A mum's heart is not made for this kind of loss," she said.

Backman also admitted assaulting corrective services officers while in custody.

With a criminal history dating back to 2008, Backman has spent most of his adult life behind bars.

Backman was sentenced in 2011 for assaulting a woman who had been on the same train before forcing her into her car and ordering her to drive.

She got help by stopping at roadworks monitored by police.

Barrister Bernard Reilly said his client used drugs daily from the age of 14 and has an "appalling criminal history" after a dysfunctional childhood during which he was exposed to violence as a witness and victim.

But in the last few years Backman completed schooling and wrote a book about his life, "more for his purposes than others'".

Mr Reilly said Backman asked to be in solitary confinement and wanted to remain in isolation as it was only there he stopped taking drugs.

"He's now had ... a clarity of thought lacking in his life from when he was a child," Mr Reilly added.

Backman realised he had ruined lives, been ruthless and reckless and made poor choices and apologised for "wicked and lawless behaviour".

Justice Paul Freeburn sentenced Backman to 15 years behind bars, saying he had committed "terrible crimes with tragic consequences".

Backman will have to serve 80 per cent of the sentence and the five years he has already spent behind bars will be taken into account.

Australian Associated Press