Kingi had been abusing people along the city's main street, Victoria St, before spying Mr Donker in his black Subaru which he later told police he'd hoped to sell for $10,000.
The court heard he'd spotted Mr Donker, who weighed just 48kg, in his Subaru and told an associate he was going to kill him and take off with his car keys.
He asked Mr Donker for food and money, but he replied that he didn't have any and went to the toilet.
Kingi yelled and kicked at the toilet door and when it opened, he punched Mr Donker in the face and hit him on the head with bottle of Jim Beam. 4kg.
He stole his car keys out of his hand as he lay unresponsive on the toilet floor.
Kingi drove off towards Auckland but was stopped at the foot of the Bombay Hills.
Mr Sutcliffe said before going into the army, Kingi was a talented sportsman and singer, and with his estranged wife, visited resthomes to sing to the elderly. However, his personality changed with the onset of his mental illness and he ended up on the streets, not taking his medication.
Justice Wylie accepted the murder was not the worst of its type and dash cam footage inside the car showed him admitting that he had "wasted" Mr Donker but said that he didn't want to kill him.
"The victim was dealt a single blow to the head with a bottle. The bottle was unquestionably brutal and callous but not to a high degree."
In 2004, Donker was jailed for more than four years for sexual offending against young boys over a 13-year period.
Victim impact statements from his sisters, who have name suppression, acknowledged his previous offending but they agreed that he had served his time for what he had done.
One hoped that Kingi would get a lengthy jail term, while the other forgave him and wasn't angry, but devastated at the way her brother had to die.
