Donaldson Samuel was one of the three men who furtively slipped aboard the 65ft ketch as it was moored overnight off the paradise island of Barbuda.
Yesterday he claimed in court that the intention of the three was robbery.
But in less than 15 minutes, the two British crew of the ComputaCenter Challenger and its two middle-aged American guests, were dead.
They had been trussed up, tortured and finally shot in cold blood after the killers failed to find a substantial amount of cash they believed was hidden on board the boat.
Rex Mackay QC, prosecuting in the case at Antigua's High Court, described the killings of ship's captain Ian Cridland, deck hand Thomas Williams and the guests, Kathy and Bill Cleaver, as a senseless, savage and sadistic execution".
Unemployed Samuel, 23, yesterday changed his original plea of not guilty to an admission of guilt to manslaughter. It was accepted by the prosecution, who immediately put him into the witness box.
In an outline of his evidence last night, Samuel claimed he and the two other accused, 23- year-old Mellanson Harris and 22-year-old Marvin Joseph, planned " to get some money at gunpoint".
He told the court: "It was all Marvin Joseph's idea."
The three young men stole a small rowing boat and paddled out to where the luxury ketch was moored, in Low Bay, Barbuda - where the Princess of Wales took a holiday last month.
Samuel told the court: "We crept on board the bigger boat and Marvin had the shotgun. He said he'd stick up the captain."
Over the next 15 minutes the three men tied up the four sailors, hands behind their backs, with their mouths taped. Samuel told the court how he searched the Challenger's cabins and found a video camera and a small amount of cash.
He added: "At that time I was on my own and that's when everything bad happened. The crew were sitting around the table when I last saw them, they were healthy, still alive. I was ready to go back because we had finished what we'd come for, we just wanted money and I wanted to go.
"Then when I went back in they were all dead. They were shot. Shot by Mel and Marvin."
In a heated exchange with defence counsel, Clement Bird, Samuel denied he had changed his plea because he was afraid to die and wanted to escape the death sentence. "Everybody has to die someday and I'm not afraid," Samuel told the court.
Samuel claimed he changed his plea because he never had any part to play in the actual killings. He said he never realised the planned robbery of the Challenger and its occupants was to be "with violence".
The trial continues.Reuse content