A jury must decide whether a man struck out in self-defence during an “altercation” that led to another man's death, a judge has said.

Andrew Boag died after an incident between him and Stephen Lowe in Garsmouth Way, Watford, on July 25, 2008.

Lowe denies murder.

Summing up the trial at St Albans Crown Court today, Judge Philip Clegg said there was “no doubt” who killed Mr Boag or how it was done.

He said: “This defendant did it by punching him with his fist on which he was wearing a knuckle-duster. None of that in dispute.”

What is in dispute, and central to the case, he said, is whether Lowe acted in self-defence.

Addressing the prosecution case, he told the jury that although Lowe may not have intended to kill Mr Boag, he may have intended to cause serious bodily harm and that is all that is needed for him to be convicted of murder.

He added: “The defence's case is that he was defending himself at all times from what he believed to be an immediate threat of being stabbed.”

Judge Clegg proceeded to inform the jury of how they should reach their verdict by spelling out the points of law they should use to conclude whether Lowe is guilty of murder or an alternative charge of manslaughter, or whether he should be acquitted.

Discussing the “main players” in the case, Judge Clegg told the jury Andrew Boag, 35, was a user of crack cocaine and a supplier of cocaine to people in north Watford.

The court heard Mr Boag had a number of convictions, which “all arise from violent or threatening incidents”, but it was an assault on Teri Leighton in 2007 that caused him to fall out with Lowe.

The seven women and five men in the jury also heard that from January 24, 2007 to June 8, 2007, Mr Boag met with psychologists and psychiatrists and was diagnosed as having a “serious personality disorder that manifested itself in a delight in violence”.

Judge Clegg said: “It gave him, to use Boag's own words, a huge adrenaline buzz. He told the doctors he enjoyed acting as an enforcer for others. Originally he had used his fists but more recently had taken to use a knife. He said he would not go out without a knife and would not hesitate to use it.

“He said he was aware those who he attacked would be unlikely to press charges against him because they were frightened of him.”

The court heard Lowe, 42, was, like Mr Boag, a user of cocaine and crack cocaine, but was not a dealer. He also had a number of previous convictions, the court heard, which included two cases of threatening behaviour and in September 2006, he was cautioned for an “unpleasant assault” on a man he said had robbed his niece.

Judge Clegg said: “The prosecution say these convictions taken together show the defendant as a man with a short fuse and has a tendency when angry to threaten unlawful violence or use it to right what he perceived to be a wrong that had been done to somebody he was fond of.

“The prosecution say he was out to get his revenge on July 25 for threats against Terri Leighton and her daughter.”

The judge added, however, that evidence Lowe saved a man from suicide in September 1993 shows a “kind and compassionate side to Frosty”.

Considering the background of the case, Judge Clegg told the jury that Lowe had a young son with Stacey Brennan, who was in a relationship with Mr Boag at the time of his death. But Lowe was not jealous, the court heard, as he was then with his “soul mate” Ms Leighton.

His only concern was that Mr Boag was around his son, the jury was told.

A text sent by Lowe to Ms Brennan on June 21, 2008, said: “He's not a friend or an enemy, just bad news and that's bad for Kai. I will kill for my kids, please remember that from the heart.”

The court heard that when Lowe persuaded Ms Leighton to give evidence against Mr Boag, after he had assaulted her, Mr Boag began calling the defendant a “grass” and issuing threats to kill him, Ms Leighton and her daughter.

On Friday, July 25, Lowe returned to Watford from Essex, where he was working at a fairground. He planned to give his son a giant cuddly toy, get his washing done and see Ms Leighton, the court heard.

Lowe, who had a knuckle-duster in his back pocket, arrived in Watford at around 9pm, the court heard, while Mr Boag had been drinking in The Badger pub on the Meriden estate since 7.30pm.

Lowe then arrived in Garsmouth Way to visit Ms Brennan and his son at the same time Mr Boag was completing a drug deal, the jury was told.

Judge Clegg said: “One thing you may think is fairly plain. All the evidence shows the meeting of Andy Boag and the defendant in Garsmouth Way must have been a chance meeting.”

He then proceeded to read evidence previously offered in court by eyewitnesses from the surrounding houses and flats who were alerted to a fight breaking out between Lowe and Mr Boag.

The court heard witnesses saw Lowe on top of Mr Boag in the front passenger seat of a Mercedes taxi, as they struggled together and saw punched being thrown by the “black man” (Lowe).

Evidence from a man named Paul Saunders, who spoke to police officers but “refused” to make a statement, was also read in court.

Mr Saunders saw a “black man and a white man fighting in the front of a car. The white man had the other man in a headlock round his neck with his left arm. His arms were pumping, hitting him at close range, saying 'Come on'”, the court heard.

Judge Clegg told the jury: “That evidence is there. You can act on it but do bear in mind it has not been tested.”

The jury then heard two arguments about the knife that was found in Garsmouth Way, in a bush near the Mercedes taxi during a police search three days after the incident.

Judge Clegg said the prosecution argued the knife “is a complete red herring and has nothing to do with the case”.

On the other hand, he said, Ms Brennan was shown the recovered knife in court and said it did look like the knife Mr Boag would carry about.

Judge Clegg said: “The evidence is that Andy told a number of people, including psychologists, he would not go out without a knife and yet one was not found in the car.”

Referring to the defendant, the court heard Lowe got “regular threats” by phone and once in person.

When he gave evidence, Lowe said that once he met Mr Boag in Garsmouth Way, he would not turn his back on him, fearing he would be stabbed.

The court heard that following his confrontation with Mr Boag, Lowe was driven away in a white transit van and threw his knuckle-duster out of the window near the Dome roundabout in north Watford.

Believing Mr Boag was still alive when he left the scene, he later received a phone call that informed him he had died, the court heard.

Three days after the incident, Lowe handed himself in to Watford police station.

Stephen Lowe, of Breakspeare Close, Watford, has pleaded not guilty to murder. He has also pleaded not guilty to an alternative count of manslaughter.

The trial continues.

To read daily updates from the trial, which began on Monday, February 9, click on the related links.