Ronald Penrose, 43, was attacked on his doorstep late one night last June and received up to eight blows. He died from severe injuries sustained when his head hit a wall.
Christopher Pitchford, for the prosecution at Cardiff Crown Court, said Michael Jones, 40, and three friends scoured the Penrhys housing estate near Tonypandy, Mid-Glamorgan, after two teenagers burgled the home of Mr Jones's elderly mother, Glynis.
Instead of passing information to police, the four confronted their victim outside his home after learning one of the burglars had lived there and might have been supplied with drugs by Mr Penrose.
Mr Penrose's son Jason, 15, was dragged from a car and kidnapped. He was threatened with drowning in a lake and stabbing unless he told the gang where to find the culprits and the stolen property. Although the prosecution accepted that the men had not gone there to kill Mr Penrose, Mr Pitchford said: 'They were all responsible for the terrible consequences which followed.
'We do not suggest that they intended them, but it was undoubtedly a possibility that existed from the decision to take the law into their own hands.'
The burglars had taken jewellery, cash, electrical goods and about 100 syringes used by Mrs Jones for a medical condition. They also took items of sentimental value, including her late husband's watch.
'Rather than leave matters in the hands of the police, Jones decided to take the law into his own hands,' Mr Pitchford said.
After he and friends had made inquiries, the group met in a pub and persuaded Kelly Thomas, 19, to drive them to the Penrhys estate to track down Mr Penrose.
The court was told that while Mr Jones punched Mr Penrose, the others surrounded Jason and a friend, Richard Jones, 17, as they sat in a car. The rear window was smashed and Jason dragged out. Richard fled.
Mr Penrose was left dying in a passageway near his front door while Jason was forced into Miss Thomas's car, roughed up, and driven to a nearby lake where he was threatened with drowning, Mr Pitchford said.
Although he protested that he knew nothing, the schoolboy was again threatened and told he would be stabbed, the court was told.
After more threats, Jason was allowed to go and returned home to find his father dead. Detectives arrested the four men and Miss Thomas the next day.
Mr Pitchford said Mr Jones admitted to police that he had been searching for Mr Penrose but solely to recover the property taken from his mother's home.
He claimed he punched out after Mr Penrose made a grab at his collar. Mr Penrose fell backwards, hitting his head. Mr Jones also told police Jason had agreed to get into the car voluntarily.
Mr Jones, unemployed, of Penygraig; his nephew Ian Jones, 23, unemployed, of Trealaw; Michael Thomas, 27, a roofer, of Tonypandy; and Howard Walsh, 47, unemployed, of Penygraig, deny manslaughter, affray and kidnapping. Miss Thomas, of Penygraig, denies kidnapping.
The case was adjourned until today.