A Perth teenager who stabbed his drunk step-father to death to stop the man from strangling his seven-year-old sister should not go to prison, his lawyer has told a court.

The 18-year-old, who cannot be named for legal reasons, was 17 when he killed his step-father inside their Girrawheen home in September 2016.

The teenager was due to be sentenced on Thursday but his case was adjourned. Credit:Heather McNeill

The teenager was due to be sentenced in the Supreme Court of Western Australia on Thursday after pleading guilty to manslaughter by means of excessive self-defence, however Justice Joseph McGrath said he needed more time to make his decision due to the unusual nature of the case.

The court heard the deceased man had a long history of being violent towards his family members, including his wife, two step-sons, aged 17 and 13, and daughter, aged seven.