Constable Zachary Rolfe, 30, has pleaded not guilty to murdering Kumanjayi Charles Arnold Walker during a failed arrest attempt on November 9, 2019.
The 19-year-old was shot three times after he stabbed Rolfe once in the shoulder while resisting arrest in the remote community of Yuendumu, 290km northwest of Alice Springs.
Prosecutor Philip Strickland SC told told the Supreme Court in Darwin that after the shooting Rolfe told a fellow officer: "He was stabbing me. He was stabbing you".
He suggested to the jury that Rolfe may have said those words "because he knew he had gone too far when he fired the second and third shots".
"He knew the shots were not necessary or reasonable. He knew everything he had done was captured on the body worn video," he said.
"In short he said those words to justify what he had done."
The first shot which was fired into Mr Walker's back while he was standing and struggling against Rolfe and and another officer's attempts to handcuff him is not the subject of the murder charge.
It relates to the second and third shots, which the prosecution says were not legally justified, when Mr Walker was laying on the ground with Rolfe's partner on top of him.
Mr Strickland said one of those shots "fatally damaged" Mr Walker's spleen, kidney and right lung and Rolfe fired them with the intention of killing or causing serious harm.
Mr Strickland also told the jury Rolfe did not shout anything before he fired.
"He did not shout: 'Knife - knife - knife" or "Scissors - scissors - scissors" (or) direct Mr Walker to drop them" which he was trained to do.
Rolfe and his fellow officers had also ignored a senior officer's instructions for arresting Mr Walker.
Mr Strickland said they were told less than an hour before the shooting to "gather intelligence" in preparation to arrest Mr Walker the next morning.
"The accused and other (officers) chose to ignore that operational plan," he said.
"When they left Yuendumu Station at 7.06pm, they were intent on finding and arresting him that evening."
After Rolfe shot Mr Walker he called out for his mother from the dark room where he lay bleeding onto a grubby mattress as the officers handcuffed him.
He died at 8.36pm after the officers had moved him to the local police station and "done the best they could to save his life" administering first aid.
The trial continues on Wednesday.
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