Lance corporal Tyler Jackson told a hearing at the Marines' Camp Pendleton base that he had been part of squad which kidnapped and shot 52-year-old Hashim Ibrahim Awad in Hamdania on April 26 because they believed he was an insurgent.
Another Marine and a Navy medic have already pleaded guilty to charges relating to the killing west of Baghdad, while another five indicted Marines have yet to stand trial.
In testimony that conflicted with evidence given in earlier hearings, Jackson said the squad of Marines had targeted Awad because they believed he was an insurgent who had planted roadside bombs.
Previously the court heard that Awad was a neighbour of a suspected insurgent.
Jackson, who pleaded guilty to kidnapping and conspiracy, said Awad was taken from his home and led to an existing crater from a roadside bomb.
A member of the group phoned the patrol base to inform them that they had spotted a man digging a hole to plant a roadside bomb. The group then opened fire on Awad, Jackson said.
Jackson said he aimed his rounds above Awad's head.
"I knew he was going to be shot, but I didn't want to be the one to do so," Jackson said. Awad got up and attempted to move but was shot several more times, the Marine said.
Jackson said the Marines had agreed to cover up the killing and filed false reports relating to the slaying.
Military Judge Joseph Lisiecki set a November 16 sentencing date for Jackson, who faces up to 15 years in custody, a dishonorable discharge and reduction to the lowest military rank.
The killing is one of a series of incidents that has tarnished the reputation of US troops in Iraq.

Copyright AFP (Agence France-Presse), 2006