Graeme Jarrett Vickerstaff, 47, Dean Phillip Welsh, 50, and Colin Maxwell Booth, 35, all previously pleaded guilty to manslaughter and faced a sentencing hearing on Monday.

The three men travelled to 48-year-old Mr Allan's Rivett home to purchase drugs, but when he opened the door Vickerstaff sprayed him in the face with a can of engine degreaser before beating him with a baseball bat.

The men robbed his home while a severely injured Mr Allan crawled up the street calling for help.

Mr Allan's father, Robert Campbell, told the court he was "shattered" by his son's death which he feared must have been "appallingly painful".

He said he was there to tell the three men how their "selfish and homicidal acts" to "indulge their addiction" had left the family with "lasting heartache and grief".

He despaired his granddaughter's wedding, Bobby Allan's daughter, would be burdened with sadness due to the killing.

"In a few short months I'll be walking Bobby's daughter down the aisle, something a father does, not a grandfather," Mr Campbell said.

"Every time I look at Bobby's children and grandchild, part of me aches."

In another victim impact statement, Mr Allan's former partner and mother to his children, Trish Campbell, spoke of the opportunities stolen from Mr Allan.

She said he never got the opportunity to overcome his demons with drug addiction and was robbed of the opportunity to watch his children grow.

"[Drug addiction is] the poison that runs through the veins of our society," Ms Campbell said.

She hoped his killers would turn their lives around and "fill the world with love not hate".

Mr Allan's daughter, Tara Campbell, said losing her father was like losing a part of her soul and that she would forever be left with unanswered questions.

"A few months before he died I pushed him away because I thought that maybe he would change and give up on the dangerous life he was living for me," Ms Campbell said.

"I will never know if all that waiting was worth anything because you took away his choices. He's gone and I will never know if I could have saved him, if he would have chosen me."

Mr Allan's mother, Lauren Allan, spoke of her son's free spirit, sense of humour, his honesty and his loving, warm hugs.

She labelled Vickerstaff, Welsh and Booth "evil cowards" and said she felt nothing but "utter disgust" for them.

"Those three evil monsters gave Bobby a death sentence and us a life sentence without Bobby," Mrs Allan said.

"We don't get time off for good behaviour."

The court heard Mr Allan turned to drugs after the 1990 suicide death of his brother Danny for which his mother said he carried guilt his entire life.

"[Bobby and Danny] are together now, brothers in arms," Mrs Allan said.

Vickerstaff, Welsh and Booth's sentencing will continue later this week.

The maximum penalty for manslaughter is 20 years in prison.