A convicted killer who was involved in the historic brutal slaying of a pensioner outside a public toilet when he was a teenager has, 13 years later, admitted to stabbing a man over cannabis.

William Frederick Izett was the oldest of three teenagers who pleaded guilty to causing the death of Donald Alfred Stewart in Hamilton in 2010 after the elderly man refused to hand over his car keys.

Izett was 17 at the time, while one of his co-accused, Ben Purua, was 15. The pair pleaded guilty to charges of manslaughter, while the third co-offender, Connor Rewha-Te Wara, admitted Stewart’s murder.

Stewart was punched in the head and knocked to the ground by Rewha-Te Wara in the unprovoked attack on June 27, 2010.

He was then dragged to an alleyway and repeatedly punched and kicked while the other two looked on.

Stewart, who had stopped at the public toilet as he drove home to Lower Hutt, was killed by a blow to the head so strong it ruptured an artery, and he was left for dead as his killers took his car for a joyride.

The retired draughtsman and builder’s body was found by a street sweeper in the inner city the next morning.

Izett was jailed for four years.

The “horrendously violent” death shocked the city’s residents and devastated Stewart’s daughter Tracey Watson and her uncle Gordon Stewart, who said then that he hoped the trio would turn their lives around.

Now aged 31, Izett appeared in the Hamilton District Court via audio-visual link today where, through his lawyer Mark Jepson, he pleaded guilty to three charges; possession of cannabis for sale, wounding with intent to injure and unlawful possession of ammunition.

Izett was staying at the Te Rapa Motor Inn’s emergency accommodation on February 13 this year.

He was transporting his freshly cut cannabis plants to his room about 10.50am when the victim approached and asked if he could buy some cannabis.

Izett told him to “go away”.

He went to his car, then returned with a hammer and a 20-centimetre-long knife and asked the victim if he wanted to fight.

A scuffle ensued, and for a short period the victim had the upper hand against Izett, but CCTV footage showed that ended when Izett stabbed him twice in the torso.

Judge Kim Saunders convicted Izett on all charges and remanded him in further custody for sentencing in December.