A teenager who fatally stabbed his abusive stepfather more than two dozen times to stop him "throttling" his young sister should not be sent to jail because of the tragic and unusual circumstances of the case, a Perth court has been told.

The teenager — who cannot be identified — was only 17 when he used one of his collection of Bear Grylls hunting knives to inflict 25 stab wounds on his 36-year-old stepfather at the family's northern suburbs home in September 2016.

The Supreme Court was told an argument developed and the man, who had consumed eight or nine full-strength beers, first started choking the teenager's disabled brother in the shower.

The teenager is set to be sentenced in the Supreme Court next week. ( ABC News )

He then went to the seven-year-old girl's room and put his hands around her neck, yelling "this is what choking looks like".

State prosecutor Bernard Standish said the girl's mother tried to stop her husband by hitting him over the head, but the child's face went red and the teenager went to his room, got a knife and "lunged at the deceased, stabbing him".

The man then started to attack the youth, who continued to stab him before yelling: "Oh my God, what have I done, I'm sorry."

Mr Standish said the youth then went outside and dropped the knife, before going back inside to help his mother perform CPR on the man.

The youth — who is now 18 — was originally charged with murder, but last year the State accepted his guilty plea to the lesser offence of manslaughter.

Teenager 'lived with fear', lawyer says

As the slightly-built teenager sat in the dock with his head bowed, the court was told he, his siblings and their mother had been the victims of domestic violence over a number of years, and his intent on that night was to simply stop what was happening.

His lawyer Karen Farley said her client had shown "true remorse from the get go".

"[The] sadness and appreciation of what he has done has stayed with this young man, and will stay with him for the rest of his life," she told the court.

"He has lived with fear before, during and since."

Ms Farley submitted that because of the tragic and unusual nature of the case, her client could receive a community-based sentence or a suspended jail term — saying he was unlikely to offend again.

Prison term needed: prosecutor

Mr Standish accepted the case was a tragic one and recognised the youth had witnessed violence being inflicted on his siblings, but said his response "was completely disproportionate" to what had happened.

"25 stab wounds was simply too many. There must have come a time there was some sort of appreciation that enough was enough," he said.

Mr Standish also said the victim was a son and a brother whose family loved him and was suffering because of his death.

He said while the case was tragic and akin to "the battered wife syndrome", the youth should receive an immediate term of imprisonment.

Justice Joe McGrath said one of the key issues he needed to consider was how many stab wounds would have been necessary to stop "the understandable perception" by the youth that his sister's life was being threatened.

"He was a young man of 17. What was necessary to stop an older, larger man choking his sister?" he said.

The youth had his bail renewed until he is sentenced next week.

As the family members of the victim left court, they said they hoped justice would be done.