Joseph Elliott, 19, admitted that he had struck the fatal blow but claimed he was acting in self-defence when Bob Osborne, 40, confronted him brandishing a hammer.
The victim's widow, Diane, wept after the not guilty verdict was announced. She said later: 'The justice system sucks. I could not speak or do anything for about an hour-and-a-half after the verdict.
'I have always had faith in our justice system and always brought up my children to have faith in the justice system, but what can I say after this?
'He just wanted to catch the guy. He did not think it would end like this. This man has admitted drinking, he has admitted taking cannabis, he has admitted taking LSD, he has admitted stabbing car tyres and he has admitted stabbing my husband, and yet he has walked free.'
A family friend said: 'Mrs Osborne is absolutely devastated. How would you feel? She has had her life taken away from her. He was only acting as a good neighbour.'
The Old Bailey was told that the tragedy began with Mr Osborne and his 38-year-old wife sitting in their back garden in Streatham, south London, watching a lunar eclipse on 10 December last year.
Anne Rafferty QC, for the prosecution, said the couple were disturbed by the sound of Mr Elliott, who was 'amusing himself' slashing car tyres.
Counsel said that the couple bravely decided to track down the vandal. Mr Osborne spotted Mr Elliott on a balcony and climbed the stairs to confront him, when he suffered a fatal knife wound to the heart.
Mr Elliott allegedly told police after his arrest that he had taken a cocktail of drink and LSD during the day.
He claimed in court that he panicked when Mr Osborne came at him with the hammer and lashed out with the knife in self defence.
'I did not mean to hurt him. I was frightened when he came at me with the hammer,' he told the jury.
Mr Elliott, unemployed of Streatham, south London, was found not guilty of both murder and manslaughter.
He refused to comment after the verdict.Reuse content