That was what Crown prosecutor Rebecca Christensen SC told the ACT Supreme Court on Friday, as she urged a jury to find the 17-year-old accused guilty of murder.

The boy has been on trial for the past month, having denied responsibility for the death of an 18-year-old man who was stabbed six times during a fight at the Weston skatepark.

He admits, however, that he knifed the deceased's 16-year-old cousin in the back, inflicting grievous bodily harm, during the same September 2020 melee.

None of the people involved in the fight, which erupted in response to a Snapchat argument, can be named for legal reasons.

In her closing address to the jury on Friday, Ms Christensen described the incident, which involved three groups of brawlers, as "a senseless tragedy".

She said jurors may feel sympathy or concern for the family of the deceased. The same could be said in terms of the accused, who was "just a boy", and his parents.

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But Ms Christensen said sympathy could not factor into the jury's deliberations, claiming the evidence she had presented led to the "inevitable conclusion" the accused was guilty.

She devoted a portion of her closing address to evidence the accused gave earlier this week, telling the jury it should reject his "impossible" and "implausible" story.

Ms Christensen ran through what she said was the correct sequence of events at the skatepark, where the violent early morning melee lasted just two minutes and 12 seconds.

She urged the jury to conclude that the accused had stabbed the 16-year-old victim on the passenger side of the cousins' car in the early stages of the incident.

The prosecutor said the boy had then moved to the driver's side to knife the 18-year-old while "no one was paying attention" to him, with many other participants in the fight having become fixated on a boy who was waving a machete around.

Ms Christensen told jurors the accused's account involved him finding and picking up a knife, which had been tossed aside after someone else had killed the 18-year-old with it, before using it himself on the 16-year-old.

This was not supported by any of the evidence, she claimed, pointing out that the accused claimed to have seen people fighting on the driver's side of the car as he found the knife.

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She said this could not be the case because, if the 18-year-old had been stabbed by someone else, the fight on that side of the car would have already been over by that point.

Ms Christensen cited the evidence of pathologist Johan Duflou in this regard, noting the professor believed 18-year-old could have died within 10 heartbeats after being stabbed.

She also said the accused was the only brawler linked by forensic evidence to the stabbings of both cousins, highlighting the presence of the two victims' blood inside the waistband of the pants he wore at the skatepark.

The locations of the blood also supported the proposition that the accused had put the knife down his pants after each cousin was stabbed, she said, depositing their blood separately.

Ms Christensen anticipated that defence barrister David Barrow would criticise other brawlers and try to discredit their evidence by telling the jury "what liars these people are".

While she accepted they were not "perfect angels", Ms Christensen said they had done their best to recall a traumatic event.

Inconsistencies in their versions and the imprecision of their evidence demonstrated their credibility, she argued, claiming these things proved they had not "put their heads together" and concocted a story.

Ms Christensen said they would have implicated the accused by saying they had seen him stabbing the deceased if their evidence was "all a lie" designed to pin the murder on him.

Mr Barrow gave jurors a brief overview of his closing address on Friday, saying he would argue four brawlers other than the accused could reasonably have been the killer.

He said the evidence pointed "strongly" towards one of these people in particular.

Mr Barrow alleged some of the brawlers called by the Crown to give evidence had lied about "important matters", saying there were "a few contenders for who was the most unreliable and dishonest".

He will make his closing submissions in full next Tuesday.

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