If I might chime in. I'm an Iraq combat tour vet, US Army Paratrooper. Did recruiting for a few years. Liberal before/during/after service. I'll preface this by saying the military experience will differ between soldiers.
What we have here is a very unfortunate example of straining the military to their breaking points. Over a decade of boots-on-the-ground, multiple deployments per brigade, and relying on the military to pretty much build both theaters of war into functional democratic societies is enough to burn out anyone. I'm very certain Europe has many wars that carried on for more than a decade where individuals or massive swaths of people decided to entertain themselves by raping and pillaging. I believe that's more human nature than nationalistic traits.
For the soldier himself, I feel sorry for him, but don't mistake feeling pity for him to equal condoning and celebrating such actions. It was a failure at multiple levels, but should the news be accurate (that the soldier was a special forces attachment, and thus under less oversight) it was mostly a failing at the individual level. In the same sense that soldier are given multiple classes of instruction to spot a troubled soldier (and what to do), kinda like how the civilian world has it's own suicide watch reminders and talking points, it's really hard to spot someone who's going to do something stupid unless you happen to be at the right place at the right time. This wasn't on-the-spot panicked wildfire into civilian crowds, this was cold and methodical execution, and again I pity the soldier but that absolutely has no place in the military proper, and nearly everyone in the military service I know would agree with that.
Now to address some of the opinions I've seen in this thread:
1) American Troops are from "the poorer and shall we say less educated people"
demographic.
You may find this surprising, but even the American military has standards for enlistment beyond being able to pull a trigger. Knowledge of basic algebra, geometry, reading comprehension, and vocabulary is just the first gate to get in. Engineering, logic, language, and a slew of other advanced studies are tested. I've personally pulled combat arms jobs off the table for an applicant who I felt was unsuitable mentally to stand beside my infantry brothers. I will say that the poor have a home in the military, provided they are smart enough to join, and don't have an unforgivable criminal background. I know there are a lot of Americans on both the right and left that would prefer the fighting be left to the criminals and destitute, but that would the worst thing to build a military around, and definitely wouldn't be good for our image as a first-world country. A military needs smart people at all levels, from command to the boots on the ground. The stupid people are back at home watching TV screaming about the evils of brown people, and voting as such.
2) Why did they(we) invade Afghanistan again?
A very publicly broadcasted act of destruction occurred in one of our major cities. We're very quick to anger when something like that happens on our home turf, and many people wanted an eye for an eye. Conservative folk have always bet on a large military to exist for such an occasion. We wanted to smite whoever was responsible for such acts, and Afghanistan was our lead. Hence began the Global War on Terror. All other things and reasons? Well, we never really thought that far ahead. A failing on our part. Namdrol's pretty correct on his thoughts except for saying W was the second big reason we went to war. Gore would have done the same, he just would have yanked the stick out of his ass after what happened, as opposed to W finding his voice and talking points.