The 17-year-old accused took the witness stand in the ACT Supreme Court on Wednesday, telling a jury he feared he was to blame until he learned, weeks later, who the deceased was.

"I thought that I had killed someone," he said.

Prosecutors allege he in fact did, closing their case against the boy on Wednesday after presenting evidence for nearly a month.

The boy has pleaded not guilty to a charge alleging he murdered an 18-year-old man, who was repeatedly stabbed during a two-minute melee at the Weston Creek skatepark in September 2020.

The boy has, however, admitted inflicting grievous bodily harm on the deceased's 16-year-old cousin by stabbing him in the back.

Called to give evidence as the defence case commenced, the alleged killer described consuming alcohol and cannabis to the point he felt "pretty drunk and stoned" prior to the brawl.

He told the court he had only been informed there was to be "a fight" when he hopped in the back seat of a car headed for the skatepark.

The boy said he became "kinda scared" and "a little bit nervous" when he saw two other vehicles arriving at the scene.

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"I just took a deep breath," he said.

"Like, I'm here now. I'm not gonna just sit in the car."

The boy told the court the melee was already in motion by the time he tried to join in, but his initial efforts did not achieve much and left him feeling "stupid".

He said he therefore walked around to the front of the car the cousins had arrived in.

"That's when I seen the knife," he said, telling the court he picked up a black-handled blade from in front of this vehicle.

The boy said he then used the blade to stab one of the rival brawlers in the back because that person was "bashing" one of his friends.

He said a bit of flesh came off this person's back, which "kind of freaked me out a bit".

"I thought, 'Oh, gross, you know'," he said.

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The boy said he went on to thrust the knife through the passenger door of the cousins' car multiple times, believing he was hitting someone inside.

He said he eventually returned to the car he had arrived in because he thought "they've had enough".

In response to questions posed by his barrister, David Barrow, the boy described being "hyped up" during the subsequent drive to a nearby house.

He said one of his fellow brawlers had filmed a video, during which he showed the knife to the camera.

The boy eventually went home to bed, putting the blade in his wardrobe because he did not want his parents to see it.

He gave evidence that he was woken by his father, who told him someone had been stabbed to death at the skatepark.

"I was pretty shocked," he said.

The boy told the court he responded by searching for information online and looking at the knife, noticing there was blood on it.

When he later walked to a friend's home, he threw it down a drain.

Asked why, he replied: "I think it was just, like, panic, really. There's been a murder. I think I've done this. Like, I'm getting rid of this."

The boy, who said he weighed about 53kg at the relevant time, later detailed being pressured to turn himself in and tell police he had stabbed the deceased in self-defence.

He told the court another brawler, said to have weighed 114kg, had manhandled him.

"[This brawler] said, 'You're gonna take the rap for this'," the accused told the court.

He indicated the same brawler also told him the deceased was someone who had been armed with a machete during the fight.

But he said he learned, after being charged with murder, that the deceased had in fact been the driver of the cousins' car, not the person with the machete.

"I didn't touch the driver," the boy said under cross-examination by Crown prosecutor Rebecca Christensen SC.

The accused, who was 15 at the time of the incident, is set to continue giving evidence on Thursday.

None of the people involved in the fight can be named for legal reasons.

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