Captain Alex Pickands told the court at a US army base in Baghdad on Tuesday that the actions of the soldiers could not be justified by the strains of war.
As the court heard closing arguments in the case, Pickands said: "Murder, not war. Rape, not war. That's what we're here talking about today. Not all that business about cold food, checkpoints, personnel assignments.
"Cold food didn't kill that family. Personnel assignments didn't rape and murder that 14-year-old little girl.
"They gathered together over cards and booze and came up with a plan to rape and murder that little girl."
The case has outraged Iraqis and led Nuri al-Maliki, the Iraqi prime minister, to call for a review of the immunity of foreign troops from prosecution under Iraqi law.
The court heard earlier how troops were "driven nuts" by combat stress and how one of the accused burned a puppy.
Justic Cross, a US soldier, described how conditions "pretty much crushed the platoon", which lived in constant fear of being killed in the Mahmudiya area south of Baghdad where the rape and murders took place in March.
He said: "It drives you nuts. You feel like every step you might get blown up. You just hit a point where you're like, 'If I die today, I die'. You're just walking a death walk."
Cross said that soldiers drank Iraqi cough syrup and whisky and took drugs to fight stress due to sustained combat.
On Monday, the court at a US base known as Camp Liberty heard graphic testimony of how three of the soldiers took turns raping the 14-year-old girl before murdering her and her family.