Constable Zachary Rolfe told his trial he feared for his life when he realised the 19-year-old was holding a "blade" in one hand and reaching for his gun with the other.
Rolfe, 30, has pleaded not guilty to murdering Mr Walker, 19, during a failed arrest attempt in Yuendumu, 290km northwest of Alice Springs.
He fired three shots into the teen's back and torso on November 9, 2019 after he was stabbed with a pair of scissors.
Asked by prosecutor Philip Strickland SC if he knew the second and third shots fired at point-blank range would probably kill the teen, Rolfe replied: "I knew it was a potential outcome".
However, he denied knowing the shots "into an area containing the teen's heart and lungs would ultimately kill" Mr Walker.
"Did you think they would cause him serious harm?" Mr Strickland replied.
"Possibly at the time," the constable told a packed Northern Territory Supreme Court on Thursday.
Earlier, the former soldier, who served in Afghanistan, said he did not initially regard the teen as a threat when he spotted him inside his grandmother's home.
That changed in a second after Rolfe and Sergeant Adam Eberl identified Mr Walker and attempted to handcuff him.
"Kumanjayi started resisting. He raised his arms and started striking me around my head and neck area," he said.
Rolfe said the teen struck him twice on the top of the head in a "hammer fist motion".
"I looked at his hands. I saw him holding a blade in a dagger-like grip. I immediately feared for my life," he said.
He said Mr Walker then stabbed him in his left shoulder and he instinctively jabbed the teen in the face and reached for his firearm.
"His left hand was already on my Glock. I twisted my hips back as we are trained to do to knock that hand off my Glock and stepped back," he said.
"Kumanjayi's focus turned to Eberl and I immediately feared for Eberl's life. Kumanjayi started stabbing Eberl in the chest and neck area.
"I drew my Glock and when it was safe to fire ... I fired one round into the centre of mass of Kumanjayi ... and was able to confirm that it hit the target."
Rolfe said the shot did not incapacitate the teen, and he and Eberl fell to the ground and continued fighting.
"I could see Kumanjayi's right arm with the blade in it still moving and stabbing Eberl," he said.
He then moved towards the men and put his left hand on his partner's back.
"I did this because they were still fighting. They were still moving. I then fired two more rounds," he said.
Rolfe said Mr Walker's right arm stopped moving after he was hit but he still believed the teen was a threat and holding a lethal weapon.
He agreed with Mr Strickland that the teen was "potentially" in extreme pain after the shots as he lay face down moaning and calling for his mother.
The court has heard constable fired his first shot about a minute after finding Mr Walker at his grandmother's home. Shots two and three followed a few seconds later.
He died about an hour after Rolfe's second shot ripped through his spleen, lung, liver and a kidney.
The Crown has conceded the first shot, which was fired while Mr Walker was standing and wrestling with Sgt Eberl, was justified.
But it says the second and third shots, which are the subject of the murder charge, went "too far".
During cross examination, Mr Strickland repeatedly asked Rolfe if he "enjoyed the adrenaline and the excitement" of high-risk arrests to which he replied: "No, it was work".
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