I agree with what you are saying, Rictor. As I have said, there is a difference between a soldier and a murderer. But even a soldier can become a murderer when he steps outside of his role, when his killing exceeds the realm of combat, duty, and conventions of warfare and enters the realm of personal pleasure, compulsion, and appetite.
A soldier must learn to come to peace with the killing in his job. I agree with you 100% on that point. I will not debate the morality of warfare here, that is not my point, but regardless, if a soldier is to remain a soldier and not go psychotic or become suicidal, he must find a way to make what he is doing fit into what he values. I don't fault soldiers for that at all. I am glad that they are able to do it. They have a hard job, and I respect them for what they do.
Getting back to the point, it is not the fact that that Private Green had a relaxed attitude about killing that is the problem or that is disturbing to me. As I said, many soldiers have come to peace with their killing due to concepts like "justice," "defending freedom," "self-defense against a malignant enemy," and so forth, and these are concepts I believe in. Being willing to kill is not pathological. It is a surivial instinct that we all must have. If only the crazies were willing to kill, everyone sane in the world would be dead before too long.

The problem I see with Private Green was not that his attitude was relaxed. In fact, his attitude, if anything, was "hungry" or "lustful." He thirsted for blood. That is different than having a relaxed attitude about it. So, I certainly hope that it didn't sound like I was trying to say that the relaxed attitude that many soldiers develop about killing is a sign of pathology. It is what they must do to survive.
As I see it, Private Green was extremely dissapointed when his bloodlust was not satiated by his first combat kill (as was to be expected). He was left feeling bored and unfulfilled. He hadn't come to peace with it. He didn't feel relaxed by it. He didn't feel indifferent about it. He was dissapointed by it. That is a very different thing.
So, what he did next was actually sadly predictable: he escalated.
I see this sort of thing in my work with sexual addicts all of the time. The guy starts out with soft porn, then heavy porn, then perhaps some deviant porn, or he starts stalking somebody or having serial affairs, and so on and so forth until something heinous happens or he gets caught. It is all driven by a similar dynamic: the "dosage" is not enough, and somehow doing things that are increasingly outside of the realm of what is acceptable by society turns up the "dosage." It's a very compulsive type of thing.
One can only assume that what he did was stepwise. He may have tried more brutal kills against military targets, he may have become "casual" about firing with civilians in the combat zone, I don't know the details.But what he ultimately did was his final escalation: crossing the boundary between soldier and murderer in an attempt to satisfy that appetite that he had been cultivating for years. The sad thing is, even if he found this fulfilling, he would only want more after that. This sort of compulsive, addictive appetite cannot be satisfied by acting upon it. And can only be quenched by rerouting and finding adaptive ways to fulfill the needs for which the compulsion is acting as a maladaptive coping attempt.
It was a good thing he was caught when he was.
So, in all fairness: this thread was started out with the caption: "A disturbing look into the mentalities of soldiers in Iraq." To be fair and accurate, all we can say is "a disturbing look into the mentality of one sociopathic soldier in Iraq." Certainly, it is not fair to take the actions and mindset of a person who is clearly pathological and generalize it to all of the soldiers there.
The sad points made by this soldier's case can be taken, but it loses its validity when it's proper frame of reference is over-extended. I think what this punctuates is not the "brutality of American soldiers," but rather the danger of certain personality types that would seek to enter the armed forces of any nation, and the necessity of screening them out, regardless of how efficient they are at their jobs.
I think it also bears recognition that this type of personality profile is very common among captured members of terrorist groups who have been psych-evaled.