The 13-year-old and his younger brother's screams woke their parents and when they burst in, Smith stabbed the father 19 times.
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The teenager sustained three stab wounds as he ran off but he was able to get to a phone to alert police.
Meanwhile, Smith tried to bundle the mother and remaining boy into the family car but abandoned the plan when the car would not start and he heard sirens.
As was the case in the past week, he did not go quietly.
Smith fled the scene and attempted to sneak back to the Wairarapa on the train.
Smith is arrested outside a hostel in Brazil this week. Photo / AP
But before he could make it back to his home town of Carterton, police collared him and he was before the court the following day.
The court file also showed the night of the murder was not the first time Smith breached his bail.
Sometime earlier, he had cased out the new family residence and hidden a .22 rifle in a hedge on an empty section next door.
On the night of the murder he retrieved the firearm and used it to threaten the mother and child during his failed kidnap attempt.
Documents added further detail to how Smith groomed his young victim too, impressing him with his motorcycle before initiating a slew of progressively more violent sexual offending.
Smith makes his escape at Auckland Airport. Photo / Supplied
"The complainant was apparently extremely fearful, his fear being reinforced by knowledge that the accused was an exponent in martial arts and had access to firearms," a pre-trial decision said.
At sentencing, Justice Greig also referred to the defendant's years of karate training.
He highlighted not only the brutality of the murder but the way he stopped the rest of the family from helping the man who they could hear moaning in another room as he bled to death.
"You prevented them from obtaining any assistance for the dying father and you took them away. All of that speaks of a cool, deliberate plan, either to retaliate against the accusations of sexual abuse that had been made or perhaps to prevent the proceeding against you," the judge said.