I think it's really interesting that you bring up the idea of shame with respect to the soldiers who participated in the killings at My Lai--also that you contrast their shame with the obvious lack of regret or remorse displayed by many of those responsible for the lynchings and other acts of violence perpetrated against black Americans by white Americans. In the famous Emmit Till case, two white men from Mississippi were charged with the brutal murder of 14-year-old Till. Emmit was from Chicago, visiting his grandfather for the summer, and didn't take seriously the warnings he was given concerning Mississippi's reputation as one of the most dangerous states a black person could find themselves in, should they run afoul of a white person. Despite the evidence presented in the trial--including the fact that Emmit's grandfather could positively identify one of the men as having been part of the group that took Emmit from his house in the middle of the night, the last time he was seen alive--the jury, all white, found them not guilty. Several months later, a magazine article/interview appeared in which the men not only admitted to having committed the murder, but freely discussed the morbid details of the event. Talking about how Till's sexual comments about white women made them decide to actually murder Till (rather than just beat him), one of the men--J.W. Milam--said, "Me and my folks fought for this country, and we got some rights." The article gives no sign that invoking his country to explain his actions caused Milam to experience any measure of cognitive dissonance whatsoever. (the article itself can be found here:
http://www.pbs.org/wgbh/amex/till/sfeat ... ssion.html)
In terms of shame and "what we are capable of," what strikes me in comparing this case or that of the Nazis with My Lai is that the American soldiers responsible for killing the unarmed Vietnamese were not psychologically insulated from the horror of their actions by the perception that their victims were somehow
less than human. The effects of "dehumanization" have been studied extensively and research suggests that it is precisely "what we are capable of" that changes as a result of dehumanizing a person or group. To Nazi soldiers who could otherwise enjoy classical music and love their children, Jews were outside of the bounds of "the human;" likewise for blacks in the southern United States and the church-going whites who murdered them. Even the simple fact of wearing a military uniform--or any uniform, for that matter--makes it easier to see another person as something else, something not quite a "person," and makes violence more feasible psychologically. I think what we saw in the video was the fact that, for the soldiers at My Lai, the dehumanized form of "the enemy" crumbled as they came to realize that the violence they were engaged in was being waged entirely from their side. Women and children, the elderly...without uniforms or weapons, and in plain sight rather than "hiding in the jungle, attacking and then disappearing," these people would have been extraordinarily difficult for the American soldiers to kill with the same readiness of mind that typically allows for taking a human life in military contexts. We are capable of murder...probably any of us is, given a sufficiently dehumanized Enemy as a target. Such is the power of something like racism or nationalism (or even an institutionally-reinforced capacity for obedience) over our "rational capacities." Personally, I simply can't imagine what it must have been like--and I empathize with those soldiers who do show remorse for what they did--to have been in their shoes when the "agentic state" provided by military combat dissolved, the situation failed to convince these men that they were simply "fighting an Enemy," and they were forced to confront the fact that they had killed innocent human beings.
If we were honestly to admit that we are capable of doing anything, what exactly would we have to admit to? You mentioned good ol' boys and "our country's values," but our country and its values are just what J.W. Milam said he had in mind when we decided he'd make Emmit Till into an example for blacks in the south. If these are the kinds of things we're capable of--and people like Sarah McCarthy are saying the kinds of things they are about schooling, obedience, and authority--I don't know, it seems like maybe "school" could stand to learn even more from history than we as individuals could. Unfortunately, school is not a sentient being capable of reflexive thought...so I guess that leaves the responsibility to us.