I agree with your Marine friend. I've never been deployed but spent 20 years in the National Guard infantry. Yes soldiers are trained to show restraint, respect the laws of war etc etc and should be held to a certain standard. But let's get real. We are sending 18 - 24 year old young men into a high adreniline, high testosterone, dangerous environment where they must kill or be killed. When they come within micro seconds of death and then realize it is their enemy not themselves lying face down in the dirt; some of them are going to over react. It is unfortunate and some discipline is in order but it should not be treated as more significant than it is. I would hit them each with a Company Grade Article-15 (goes in permanent service record) which can take up to three months pay and reduce junior enlisted by one grade in rank. If any NCOs were involved, a Field Grade Article-15 might be in order (Take 6 months pay and reduce by two grades in rank). Nothing more is needed.
As for the impact on national security, Nicolo Machiavelli was correct. If you must choose between being feared and being loved; choose to be feared. I would rather our potential enemies worry about what our soldiers might do to them after death than consider our soldiers easy marks. When Lyndon Johnson first spoke of "winning hearts and minds" the soldiers in Vietnam responded with signs that said "When you have them by the b--ls, their hearts and minds will surely follow." Sometimes the PFC grunts have more wisdom than the Commander in Chief.