The decision came in the case of Staff Sgt. Robert Bales, 40, who pleaded guilty in June in a deal to avoid the death penalty.
Bales did not recount specifics of the horrors in court when he testified Thursday or offer an explanation for the violence, but he described the killings as an "act of cowardice, behind a mask of fear, bulls--- and bravado."
"I'm truly, truly sorry to those people whose families got taken away," he said in a mostly steady voice during questions from one of his lawyers. "I can't comprehend their loss. I think about it every time I look at my kids."
Bales said he hoped his words would be translated for the nine villagers who traveled from Afghanistan to testify against him - none of whom elected to be in court to hear from him.
His statements were not made under oath, which prevented prosecutors from cross-examining him.
Bales, a father of two from Lake Tapps, Wash., was serving his fourth combat deployment when he left his outpost at Camp Belambay, in Kandahar province, in the middle of the night to attack two villages.
The nine Afghans - some angry and at least one cursing Bales - testified over two days about their lives since the attacks. Haji Mohammad Wazir said he lost 11 relatives, including his mother, wife and six of his seven children.
"If someone loses one child, you can imagine how devastated their life would be," Wazir said. "If anybody speaks to me about the incident ... I feel the same, like it's happening right now."
Attorneys for Bales made much of Bales' repeated deployments and suggested that post-traumatic stress disorder and traumatic brain injury may have played a role in the killings. But they offered no testimony from psychiatrists or other doctors, saying they saw little point in making the case a battle of the experts.
Instead, they had Bales and some of his fellow soldiers testify about the difficulties they endured and the images that stuck with them after earlier tours in Iraq. They rested their defense after Bales finished speaking.
In his closing argument, the prosecutor, Lt. Col. Jay Morse, displayed photos of a young girl who was executed as she screamed and cried, as well as surveillance video of Bales returning to the base with what Morse called "the methodical, confident gait of a man who's accomplished his mission."
While questioning other witnesses, prosecutors noted Bales' checkered past, including a fraud investigation and eventual $1.5 million judgment, a drunken-driving arrest in 2005, a driving under the influence crash in 2008, and lies on re-enlistment documents about his criminal history.
Bales' lawyers did their best to paint a sympathetic picture of a patriotic man who was an ideal father and had been his senior class president and quarterback of the high school football team in Norwood, Ohio.