As a thought experiment, suppose that one of these men had come home from his tour of duty, had a PTSD flashback (as has been known to happen) and started mowing down US civilians with the same indiscriminate enthusiasm? Would you continue to argue that fighting men cannot be held to the same standards as lesser mortals, and it should be dismissed as a breach of fair play?
Or, given your argument that it's the heat of battle that excuses everything, suppose he got caught up in an armed robbery, drew his piece, and then after shooting the gunmen carried on to shoot down a dozen or so innocent bystanders? Do you suppose the police would agree that anything can happen in a firefight?
Would it be more excusable if they were only Muslim immigrants, and he'd flashed back to "kill all ragheads" mode?