Omari Roberts, 23, was told by the Crown Prosecution Service that no evidence would be offered against him, the day he was due to stand trial for the murder of 17-year-old Tyler Juett.
Nottingham Crown Court heard Roberts confronted the teenager after he returned home from work for a lunch break in March last year.
He found the patio door of his mother's semi-detached house in the Basford area of Nottingham smashed in.
Roberts then grabbed a kitchen knife from a drawer to protect himself as he was attacked by Juett's 14-year-old accomplice, who it is also believed was armed with a blade.
In the struggle the youngster, who cannot be named for legal reasons, was stabbed twice in the knee and was chased off the property by Roberts.
Juett, who was upstairs ransacking a bedroom, rushed down the stairs to confront Roberts, who in a fight stabbed the teenager in the shoulder, severing a major artery.
Today it emerged Juett's accomplice was the prosecution's main witness, despite having an anti-social behaviour order banning him from parts of Basford and a string of convictions for violent offences, including burglary.
Roberts was formally found not guilty over the death today and was discharged by Mrs Justice Dobbs.
In a press conference immediately following the hearing, it emerged some police officers did not want the case to be brought against Roberts.
His mother Jacqueline McKenzie-Johnson, 47, said the law now needed to be clarified.
At present home-owners are not allowed to use "unreasonable force" but there have been calls for it to be strengthened so that only "grossly disproportionate force" would warrant prosecution.
She said: "There's a need for clarification on 'reasonable force'. I particularly believe that when you are faced with an intruder in your own home, the expectation that you behave reasonably doesn't seem to fit.
"There are number of things that have had a bearing and I am sure that the General Election had something to do with it. Also, the overwhelming support we have had from the public may have influenced the decision as well as the fact there was no evidence."
Jerome Lynch QC, Roberts' defence barrister, said he had not ruled out making a compensation claim.
He said: "When I first saw Omari I said: 'Boy, I have been doing this for 27 years and I am going to get you off this'. I have never been so sure of a case.
"My view is this: somebody has started looking at the case and thought 'Oh my God, let's speak to this boy (the accomplice) again'.
"They send two police officers to wherever he is serving his prison sentence and they get a completely different version because he can't remember what lies he told last time. They think they can't rely on him and they drop the case."
Outside court, Roberts said the experience of confronting intruders in his own home had been "horrific".
He told the press conference at the Crowne Plaza hotel in Nottingham: "The past year for me has been a nightmare. It's been very stressful. I haven't wanted the events to happen as they have happened.
"I have been taken away from my family. I have had restrictions to what I can do. I am overwhelmed that the charges have been dropped but it still hasn't sunk in yet."
Patrick Mercer, the Tory MP for Newark, said it was the Conservatives' position that the law should be strengthened to protect home-owners.
At present, home-owners are not allowed to use "unreasonable force" but he thinks this term should be changed to "grossly disproportionate force".
In 2005, Mr Mercer had his Householder Protection Bill, a Private Member's Bill, struck down.
He said: "A large number of people are arrested, cautioned, examined, charged and then when they come to court the whole thing is dropped.
"If you raise the threshold a lot of these cases would never be brought and you won't waste the court's time and, most importantly, the home-owners."
In a statement, the CPS said: "The decision was made after significant new information was received by the CPS.
"The CPS has a duty to keep cases under continuous review and when new evidence comes to light, we are obliged to consider what effect it will have on a case.
"After receiving significant new information last month, the case was reviewed and it was decided there was no longer a realistic prospect of conviction. This information was not available at the time the original decision to prosecute was made.
"We have today informed Nottingham Crown Court that we are no longer going ahead with the trial."
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