Calvin Gibbs testified at his court-martial at Joint Base Lewis-McChord in Washington state, CNN reported. He is charged with heading what prosecutors have called a "kill squad" in the 5th Stryker Brigade.
Gibbs had served a tour in Iraq and was on his second in Afghanistan when he took over the 3rd Platoon. He said his predecessor lost his legs in a bombing.
On the stand, he said another soldier called him a "sicko" in Iraq after he shot into a car at a checkpoint that turned out to hold an unarmed family. But he said his superiors did not discipline him.
He admitted cutting fingers from one of three Afghans he allegedly had killed and giving them to the soldier he believed shot the man.
"I was numb to the situation," Gibbs said. "I wasn't thinking, it's sickening. I am embarrassed."
Three soldiers from the Stryker Brigade have pleaded guilty to murder and six have been found guilty of lesser crimes. A total of 12 have been charged.