A northern Alberta teen had no choice but to shoot his violently abusive father to defend himself and save his mother's life, a judge has ruled.

At the conclusion of his trial in High Level, Alta. last month the boy, now 15, was found not guilty of second-degree murder in the death of his father two years ago. He cannot be named under the Youth Criminal Justice Act.

"I find that H. did not intend to kill his father, intending only that his father would stop what he was doing before killing his mother," Justice Paul Jeffrey said in written reasons for his decision released this week.

"I find in all the circumstances, H. had no other choice if he was to intervene to save his mother," the decision said. "In the circumstances, his actions were reasonable or, at a very minimum, he has raised more than a reasonable doubt that his acts were reasonable in all the circumstances."

The boy was 13 in August 2013 when he shot his father once in the leg with a 30/30 Winchester rifle to stop him from attacking his mother, who had just been released from hospital. She had spent weeks on life support after the boy's father nearly choked her to death.

The boy shot his father a second time in order to stop "the violent, enraged brute of a man" who was now "floundering" towards him, the judge said. He noted the father had threatened to kill the boy.

The judge believed the boy when he testified he didn't intend to shoot his father in the stomach.

After firing the gun, the teen picked up his baby sister and ran out the door.

"I concluded that the first shot taken by H. was in defence of and to protect his mother, to avoid her imminent murder if he did not intervene," the judge wrote.

"I find the second shot was in defence of himself and also his mother, because the first was insufficient to restrain his father's aggression, by that time headed towards him."

The teen was arrested several hours later. He spent more than two years in custody at the Edmonton Young Offender Centre.

The written judgment chronicles the horrific abuse the boy and his mother suffered at the hands of his father.

His mother had a number of visible permanent injuries from years of physical abuse. She had a permanent bump on her head after her husband kicked her in the head.

Whenever the boy looked at her, the judge wrote, "he saw another visible reminder of his father's abhorrent, brutal, criminal treatment of his mother."

The boy had suffered repeated beatings at the hands of his father since he was seven years old. He had been whipped with cords, struck with wooden objects and thrashed with an antenna.