Joint Base Lewis-McChord, Washington (CNN) -- An Army staff sergeant accused of leading a rogue "kill squad" charged with murdering three Afghan civilians took the stand Friday during his court martial and denied carrying out the killings. Yet he admitted to cutting off body parts as part of what prosecutors called a gruesome practice of keeping battlefield "trophies."
Staff Sgt. Calvin Gibbs' surprise testimony came a week after his court martial began. It is the first time Gibbs has publicly presented his version of what happened with the platoon he led from the Army's 5th Stryker Brigade.
He is the highest-ranking soldier charged in what prosecutors say was a rogue "kill squad" that allegedly targeted Afghan civilians and made it look like they'd been insurgents.
Gibbs has pleaded not guilty.
Twelve soldiers have been charged in the case. Three have pleaded guilty to murders and agreed to testify against fellow soldiers. Another six have been convicted of lesser crimes.
Besides the murders, Gibbs also has been charged with removing body parts from his alleged victims, such as teeth and fingers, to keep as souvenirs; planting "drop weapons" to fake attacks on soldiers; and intimidating several of his own unit members to prevent them from speaking out against the unit's alleged murder plots and rampant drug use.
Having already served a tour in Iraq, Gibbs was on his second tour in Afghanistan when he took over the brigade's 3rd platoon. Its previous leader lost a leg in a roadside bombing, an attack that Gibbs said adversely affected his morale and others.
"I should've been on the truck that day," he testified Friday.
Soldiers from his platoon testified during the court martial for Gibbs -- who stands 6 feet, 4 inches tall and has told investigators that his six skull tattoos are to track his kills in Iraq and Afghanistan -- that he called Afghans "savages," talked about how he killed civilians in Iraq and told them they should not hesitate to fire when in a difficult spot.
"If they are put in a tight situation, don't not pull the trigger," Gibbs said he advised members of his new platoon. "You won't go to jail for it."
He testified Friday about telling soldiers serving under him in Afghanistan how he "lit ... up" a car coming toward him in Iraq, while he was manning a nighttime checkpoint. Inside the car was an unarmed Iraqi family, Gibbs said.
He recalled how, immediately after this shooting, another soldier called him a "sicko" and said he would "fry" for the family's deaths. But his superiors later told him he would not face charges, Gibbs' testified.
Another soldier -- Private Jeremy Morlock -- testified that Gibbs bragged about the Iraq killings and claimed the incident showed that troops could target civilians without facing repercussions.
Morlock is serving a 24-year sentence for the killing of three Afghan civilians. He agreed to testify against other soldiers as part of a plea deal with prosecutors. Gibbs' attorney said Morlock and other soldiers who have testified against Gibbs did so to save themselves from longer jail sentences.
The defendant, himself, insisted Friday that never murdered civilians in Afghanistan.
Still, Gibbs did admit that -- in January 2010 -- he removed fingers from the body of a dead Afghan man, which is against Army regulations. The man is one of three Gibbs is accused of murdering.
He testified about later giving the fingers to the soldier that prosecutors say shot the man, in part because "people wanted to prove they were there."
"I was numb to the situation," Gibbs told the court about why he had ripped the fingers from the corpse. "I wasn't thinking, it's sickening. I am embarrassed."
Defense attorney Phillip Stackhouse showed a photo of Gibbs and two other soldiers posing with the body of a different Afghan man. Gibbs has been charged with shooting that man in February 2010 in cold blood and planting an AK-47 near the body to make it look as if the man was an insurgent.
Morlock, who is also in the photo, testified previously that Gibbs fired the weapon to make it look like they had come under attack first.
On Friday, Gibbs maintained the man had fired on the three soldiers as they patrolled a village in Kandahar Province, insisting he shot back to protect himself and fellow soldiers.
"He got the drop on me," Gibbs testified. "He engaged me with two to three rounds" before the man's weapon malfunctioned.
The staff sergeant admitted to using shears to cut off this man's finger after killing him, saying, "That was the finger he tried to kill me with, I was pretty pissed off about it."
Speaking in a calm, steady voice throughout his day-long testimony, the staff sergeant denied testimony from fellow soldiers that he goaded his men into targeting civilians and employed "off the books" weapons.
He also denied murdering a third man -- the mullah of a village, who two fellow soldiers have testified Gibbs killed with a grenade -- in March 2010. Yet Gibbs acknowledged he cut off out one of that man's fingers.
"I compared it to keeping antlers off a deer," the staff sergeant testified, saying he agreed to let his soldiers take their own "trophies." "I didn't want to disappoint them. I didn't want to come across as a pussy, which I am not."
Gibbs also admitted Friday to taking part in the beating of Pvt. Justin Stoner, who complained about other soldiers smoking hashish in his room. For good measure, Gibbs said he showed Stoner several dead men's severed fingers and told him, "Better not to talk."
But Stoner did talk, telling investigators that Gibbs' platoon had begun making up their own missions and selecting their own targets.
Military judge Lt. Col. Kwasi Hawks said the case -- to be decided by a five-person panel including three military officers and two enlisted personnel -- should be over by the end of next week. If convicted on all counts, Gibbs could face a life sentence in military prison.
"I am terrified," the staff sergeant said Friday, when asked by his attorney about how he felt about testifying.