Derek Grant’s son Jordan, a politics student, was held up at knifepoint by Patrick Bradley as he made his way home from work last August.
However, the 20-year-old and his father later found Bradley after tracing his missing phone using a mobile device tracking app.
Derek Grant demanded the handset from Mr Bradley – who then blinded the father-of-four by stabbing him in the eye.
But Grant, who had been badly injured, retaliated by knifing the 29-year-old and fatally injured him.
It led to the 38-year-old being charged and yesterday, at the High Court in Glasgow, he admitted the culpable homicide of Bradley.
A judge heard how Grant had only reacted after being subjected to “extreme violence” and that the incident was a “tragedy” for his “high-achieving” family.
Grant was remanded in custody and will be sentenced next month.
The court heard how Jordan Grant had been returning to his home in Greenock late at night on 30 August last year when he was confronted by Bradley, who brandished a knife at him.
Bradley – whom the court heard had a significant record for violence – ordered him to hand over his iPhone, which he did.
Jordan went home and told his father who – using the “Find My iPhone” app – traced the mobile and discovered it was still close by.
Grant then left his home in the town’s Wellington Street with Jordan and his two younger sons Lee, 17, and 16-year-old Jamie.
They spotted Bradley, who was not known to them, and Derek Grant ordered him to hand back the stolen phone.
But Bradley instead lashed out at Grant and horrifically knifed him in the left eye. His frantic son Jordan then screamed for help.
His father then repeatedly struck out at Mr Bradley with a knife he had taken from home, having feared for his safety.
Bradley suffered a cardiac arrest and died following the incident, the court head.
Grant was arrested and he later said: “I wish to declare for the record that, at the time of this incident, I was acting in self-defence.”
Grant had faced a murder allegation before prosecutors yesterday accepted his guilty plea to the reduced charge of culpable homicide.
His three sons were also accused of murder, but their not guilty pleas were accepted.
Advocate depute Douglas Fairley QC said it was agreed that Grant had suffered a “significant” degree of provocation before the fatal assault on Mr Bradley.
Grant’s lawyer yesterday told the court that his client only lashed out as a “reaction to the extreme violence” he faced.
Ian Duguid, defending, added that what happened had been a “tragedy” and “catastrophic” for a family that were law-abiding.
The court heard that Jordan Grant was a university politics student, another son had hopes of being a footballer, while a third was doing well at school.
Mr Duguid went on: “It may become evident this is a family of high achievement.”
The QC also said Grant had been forced to give up his job as a driver, having lost the sight in his eye.
Lord Boyd remanded Grant in custody and deferred sentencing until 1 September at the High Court in Livingston.