Two brothers tried to kill a man they thought had been violent towards their mother.
Christopher, 26, and Darren McLaughlin, 23, stabbed Kevin McCartney at his flat in Alexandria, West Dunbartonshire in October 2010.
They attacked after spotting marks on the neck of their mother Isobel McLaughlin and assuming that her partner was responsible.
The 48-year-old victim, who was a qualified intensive care nurse and had been convicted of domestic assault in the past, later died.
The brothers appeared at the High Court in Glasgow on Thursday accused of murder.
The Crown accepted their guilty plea to charges of attempted murder, after the court heard that Mr McCartney died due to "catastrophic" medical treatment.
Their mother had also been accused of murder but her not guilty plea was accepted.
The court heard how Mrs McLaughlin, her partner and her sons had all been at the same local bar on October 30, 2010.
She was seen arguing with Mr McCartney before he left the bar to go home. He phoned the police in the early hours to say his partner's sons were kicking the door of his flat.
He was stabbed a total of 13 times on the chest and back.
Mr McCartney was taken to Royal Alexandra Hospital, before being transferred to Glasgow's Southern General Hospital.
His condition worsened after medics failed to spot a perforation to his bowel, and more than a week alter he was found to have fluid in his lung.
Within minutes of a chest drain being inserted, Mr McCartney collapsed and died.
Staff reviewed the incident and found the chest drain to be "inappropriate" and that there had been "several missed opportunities".
The brothers have been remanded in custody before they are sentenced next month.