"The concept ... is more an existentialist one," she says.
The Marines, who prefer to call moral injuries "inner conflict," started a few years ago teaching unit leaders to identify the problem. And the Defense Department has approved funding for a study among Marines at California's Camp Pendleton to test a therapy that doctors hope will ease guilt.
But a solution could be a long time off.
"PTSD is a complex issue," says Navy Cmdr. Leslie Hull-Ryde, a Pentagon spokeswoman.
Killing in war is the issue for some troops who believe they have a moral injury, but Ritchie says it also can come from a range of experiences, such as guarding prisoners or watching Iraqis kill Iraqis as they did during the sectarian violence in 2006-2007.
"You may not have actually done something wrong by the law of war, but by your own humanity you feel that it's wrong," says Ritchie, now chief clinical officer at the District of Columbia's Department of Mental Health.
Kudo's remorse stems in part from the 2010 accidental killing of two Afghan teenagers on a motorcycle. His unit was fighting insurgents when the pair approached from a distance and appeared to be shooting as well.
Kudo says what Marines mistook for guns were actually "sticks and bindles, like you'd seen in old cartoons with hobos." What Marines thought were muzzle flashes were likely glints of light bouncing off the motorcycle's chrome.