"His actions caused my son's death... That's justice for you New Zealand. The law's an ass."
A 16-year-old boy previously came before the High Court after also admitting assaulting Stephen.
He too was discharged without conviction earlier this year.
The 18-year-old was to have faced trial for manslaughter but in June pleaded guilty to an amended charge of assault with intent to injure.
Brent Dudley delivers his victim impact statement. Photo / Doug Sherring
West Auckland boy Stephen died after a school rugby training session on June 6 last year.
He was rushed to Auckland City Hospital but died a short time later.
Critically, medical examinations showed an undiagnosed heart condition contributed to his death, which saw the Crown withdraw the manslaughter charge.
The court heard emotional victim impact statements from Stephen's mother, sister and particularly his father, who called the teen "the hand of evil" while staring at him in the dock.
"The actions you took were nothing but cowardice and brutality," he said.
"He was a lot smaller than you and you attacked our son from behind. He had no idea about the attack he was to be subjected to.
"Any thoughts of forgiveness are out of the question at this stage. I hold you entirely responsible for the death of our son... you own that."
Defence lawyer John Munro said his client had been excluded from school and experienced severe social isolation because of what happened.
He called it "a very very poor error of judgement" and stressed how much he had learned from the experience, giving speeches to young people at his church about the consequences.
Justice Winkelmann said there was no indication the defendant had ever resorted to violence in the past and he could not be sentenced on the basis his actions caused Stephen's death.
"You have lived and will live your life in the knowledge of your role in the events that ended with Stephen's death and I am satisfied that you do feel the weight of that," she said.
"I consider that there is a real and appreciable risk that your transition into adulthood, given your current prospects and educational ambitions, will be significantly prejudiced should a conviction for violence be entered against you."
Read more: Family close ranks over shooting