Siale Tinoai stabbed his housemate 200 times then replied "sweet as" to police who informed him Craig Stuart Field was dead.
Prior to the murder, Tinoai drew a picture depicting himself, wrote the words, "Craig, you're gonna die" on it and left it on a table in their Brisbane sharehouse.
He and Mr Field, 43, shared a house for people with mental health care needs with three others in Slacks Creek in January 2013.
Their relationship began to sour after Mr Field wrongly accused Tinoai of stealing a video game.
On the day of the killing, a woman who lived on the lower floor heard heavy footsteps above, then Mr Field yell "help, help".
She ran upstairs to see Tinoai on top of him in the lounge room and repeatedly stabbing him.
"Help me. He's trying to kill me," Mr Field said repeatedly.
Police and paramedics were called but Mr Field, a father of two teenage boys with intellectual disabilities, died at the scene.
Tinoai fled but was soon found by police, when he claimed Mr Field attacked him with scissors over the alleged theft of video games.
"Some c*** tried to stab me so I defended myself and killed the c***," he told them.
"I grabbed the scissors ... then I pushed him on the couch and went on him and started shanking him."
Tinoai, 27, who also assaulted a corrective service officer and dog while in custody, was sentenced to life in jail in Brisbane Supreme Court on Thursday.
Justice Peter Davis opted against extending his non-parole period beyond the mandatory 20 years after considering his youth, violent upbringing and borderline personality disorder.
With time in pre-sentence custody already served, he could be released in 14-and-a-half years.
Mr Field was rendered defenceless by Tinoai sitting on his upper body as he stabbed him.
"The assault upon him was excessive, indeed gratuitous, and (he had) complete disregard for the life of another person," crown prosecutor Mark Green said.
"There must have been some level of awareness on the part of the defendant that Mr Field had become defenceless and did not pose, in any way, a possible threat to him."
Mr Field died six days before a court date to determine if he was mentally well enough to get his children back from foster care.
Family members wailed as details of his death were heard.
His sister Heather Saint told the court it had put significant strain on the extended family over care arrangements for his two sons.
His son Keiran Field, who has autism and Asperger's, said he had moved from home to home since his father died.
"That was tough. I don't like that at all," he said.
Australian Associated Press