"It has had a severe and no doubt long-lasting impact on Mrs Compton as she was sitting in the passenger seat next to him when you fired the fatal shot," Justice Burns said.

Mr Compton and his wife Karen had been departing Nunns' home at Warwick, southwest of Brisbane, in the early hours of April 5, 2020 when Nunns attacked Mr Compton with a metal pole.

Nunns returned to his home to retrieve a rifle and fired at least one shot at the Comptons' vehicle before moving closer and firing a round through the windscreen.

The second bullet struck Mr Compton in the neck, severing an artery and causing him to bleed to death.

Crown prosecutor Matthew LeGrand said Mrs Compton had suffered ongoing pain from the loss of her husband and it had also affected her family's lives.

Justice Burns said the jury had heard a significant amount of evidence about Nunns' state of mind at the time of the killing, which followed him inviting the couple to his home to consume methamphetamine.

"At some point, from the wife’s account, (the couple) left to get fast food. Your evidence is that you interpreted what was occurring as an attempt to lure you away from your home and kill you," Justice Burns said.

He said the jury had considered evidence that Nunns might have been affected by schizophrenia or related disorders, possibly induced by drug consumption, but had found Nunns did not reasonably believe he was acting in self-defence.

The jury also found that Nunns did not have diminished responsibility at the time and had intended to kill or cause grievous bodily harm to Mr Compton.

"There is only one sentence that I can impose as you well know by now. On count one of murder you are convicted and imprisoned for life," Justice Burns said.

He imposed a non-parole period of 20 years with 1529 days on remand declared as time served.

Nunns did not visibly react to the sentence and spoke briefly with his defence barrister before being returned to custody.