"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.
"There's no day — whether it's in the shower or whether it's walking down the street ... that I don't think about things that happened over there," says Kudo, now a graduate student at New York University.
"Human beings aren't just turn-on, turn-off switches," Veterans of Foreign Wars spokesman Joe Davis says, noting that moral injury is just a different name for a familiar military problem. "You're raised on 'Thou shalt not kill,' but you do it for self-preservation or for your buddies."
Kudo never personally shot anyone. But he feels responsible for the deaths of the teens on the motorcycle. Like other officers who've spoken about moral injuries, he also feels responsible for deaths that resulted from orders he gave in other missions.