Thomas Tihema Christie was found guilty in the High Court at Christchurch last month of the manslaughter and indecent assault of Shaun Finnerty-Gallagher whose near naked body was found in Upper Riccarton's Auburn Reserve in February 2008.
Justice Graham Panckhurst rejected defence counsel Margaret Sewell's suggestion that Christie did not have a "Fagin-type of personality" and was not using stand-over tactics to make the victim steal alcohol.
The judge interrupted Ms Sewell to cite the evidence of the video from a liquor store on the night the youth died, showing Christie selecting liquor and giving it to the youth to steal.
Miss Sewell maintained Mr Finnerty-Gallagher was a willing participant in the theft, which resulted in him being tackled in the car park afterwards.
She spoke of 26-year-old Christie's deprived background, losing his parents when he was young and later living hand-to-mouth on the streets and stealing alcohol.
He had felt the death of Mr Finnerty-Gallagher deeply but did not know how he had brought it about in the night time drinking session at the reserve in February 2008.
Christie's long list of convictions included disorder and minor dishonesty, but it also listed two robberies in 2004, an assault in 2005, and another robbery in 2006.
Marty Finnerty, the dead boy's father, said in his victim impact statement which was read in the court: "Please help the street kids by taking an evil monster off the streets for a long, long time."
Justice Panckhurst said it was clear from the evidence that members of the group had seen Mr Finnerty-Gallagher, who was young and had a slight build, as being "fair game". He had attention deficit hyperactivity disorder and had repeatedly run away from home and social welfare and fallen in with the group.
On the night of his death Christie had taken him on an alcohol raid at a liquor store, and had later sent him to steal petrol. He broke his nose with a backhander at the park when he failed.
Justice Panckhurst noted that Christie had written a letter to his victim's family, but he had been unable to see any obvious sign of remorse during the trial. However, he accepted that Christie did not envisage or intend the youth's death.
He imposed a minimum non-parole term of three-and-a-half years on the six-year sentence.
