Alex Fisher's body was found on Waitarere Beach on October 8 after his disappearance sparked a huge three-day manhunt.
During the search, police released CCTV footage showing Alex and McIsaac together in a local store on October 5.
McIsaac was arrested and appeared in court on October 7, initially charged with possessing a knife in a public place. He also pleaded guilty to that yesterday.
A Waitarere father and his adult son were part of a group searching for Alex when they came across McIsaac on a road among trees surrounding the town, near Levin.
"My son and I walked him out. He was just standing there and we told him there were people looking for Alex. He seemed to act surprised and mumbled something like, 'Why, what for?'," said the father, who thought McIsaac wasn't telling the truth but didn't want to upset him.
"My son said to me after, 'Dad, did you know he had a knife?' I didn't, but my son had a stick and held on to it and said to me he was waiting for the knife to come."
McIsaac is no stranger to the justice system. He was jailed for two years and seven months in 2012 for setting fire to his father Peter McIsaac's Wairarapa house, putting a sleeping neighbour's life at risk.
In the early hours of the morning, McIsaac cut holes into his dad's walls, stuffed them with paper and poured oil over it.
He lit a piece of paper using a toaster and the flames quickly spread, destroying his dad's place and the neighbour's adjoining unit too.
The Masterton District Court heard McIsaac was aware the neighbour, a 60-year-old woman, was inside, but did nothing to help her. She managed to escape.
"See this man not as a vindictive, bitter young man who set fire to his dad's house because he was angry with him, but this is a young man who has struggled within his family relationships for many years," McIsaac's lawyer said at his sentencing.
McIsaac has also admitted a charge of burglary. He will stay in custody until sentencing in July.
