"You stop looking at people as people," Cpl. Trent D. Thomas, 25, told CNN in a segment aired Monday night.
The interview was filmed Feb. 7 in the Camp Pendleton office of Thomas' military lawyer. The next day, Thomas withdrew his guilty plea to murder, kidnapping and other charges.
A squad of seven Marines and a Navy corpsman were charged with killing Hashim Ibrahim Awad, a 52-year-old retired policeman. Four have pleaded guilty to lesser charges.
On Tuesday, one of the other Marines charged in the case, Lance Cpl. Robert B. Pennington, 22, was scheduled to appear for his court-martial at Camp Pendleton.
In the CNN interview, Thomas declined to say whether he was following orders, but the next day he told a military judge that he was following instructions from his squad leader, Sgt. Lawrence Hutchins III, and an unidentified lieutenant.
Hutchins is awaiting trial on murder and other charges. He has not yet entered a plea, although his lawyer has said he did not think Hutchins did anything wrong.
Thomas, in the interview, showed conflicting sentiments about the killing, the AP reports.
"At the time, I felt that I was doing what I had to do," he said. "Now that I'm back here, I know that it was wrong what we did, and for that I am truly sorry."
According to prosecutors and testimony from the four squad members who have pleaded guilty, the troops wanted to kidnap and kill a suspected insurgent. When they could not find him, they instead seized Awad, prosecutors alleged.
Thomas told CNN he remained committed to the Marine Corps.
"I'm not a hero, I'm not a murderer, but I'm a Marine," Thomas said.