Staff Sgt. Robert Bales apologized for massacring 16 Afghan civilians, calling the rampage an "act of cowardice" and asking a jury of soldiers for leniency in his sentence.
Bales, 39, said he conducted the March 2012 late-night attack in which he alone slaughtered men, women and children "behind a mask of fear ... and bravado."
"I'm truly, truly sorry to those people whose families got taken away," he said, according to The Associated Press.
The jury, impaneled at Joint Base McChord in Washington, will deliberate over whether Bales' life sentence should offer the opportunity of parole.
Defense lawyers have tried to depict Bales as a loving father, affected by post-traumatic stress disorder after multiple tours in Afghanistan, and a traumatic head injury. The soldier's brother, and an old friend, former pro footballer Marc Edwards, testified on his behalf.
"There's no better father that I've seen," William Bales, 55, said of his younger brother. "If you brought the kids in here today, they'd run right to him."
Bales has never denied leaving Camp Belambay in Kandahar province and walking by himself to two neighboring villages where he fired on locals with a 9mm pistol and an M4 rifle.
Bales pleaded guilty to the charges against him in June and confessed to the murders to avoid the death penalty.
In June, Bales pleaded guilty to murder, attempted murder, aggravated assault, as well as illicit use of alcohol and drugs.
When the judge, Col. Jeffery Nance, asked Bales why he carried out the rampage, Bales said: "I've asked that question a million times, and there is not a good reason in the world for the horrible things I did."
Earlier this week, prosecutors called on testimony from Haji Mohammad Wazir, a villager who lost 11 family members in Bales' rampage, including his mother, wife and six of his seven children.
"If someone loses one child, you can imagine how devastated their life would be," said Wazir. "If anybody speaks to me about the incident ... I feel the same, like it's happening right now."