Why do people who are so nice, elect such knob-heads and allow their hard-working, brave, young soldiers to commit atrocities?
Well, as to the first question, many would say that the knob-head wasn't actually elected but appointed by the Supreme Court initially. And there were certain shenanigans regarding the 2004 election as well. But even if one believes GWB was elected legitimately, he held the slimmest of majorities and there was tremendous opposition to him.
My friends in the UK are wonderful people, and yet the UK has also had its share of appalling Prime Ministers over the years (e.g., the late Margaret Thatcher, horrible Tony Blair, et al).
As for the second part of the question, the sad truth about human nature is that placed in the wrong circumstances, many people can behave very badly, up to and including committing atrocities. For a more benign example, see the Stanford prison experiment. For a glimpse into some of the worst of human nature, there's a brilliant book called "Ordinary Men."
If the institutional culture allows for and even approves of violations of the most basic rules of human decency, people will follow suit. After 9/11, the US government made some very dangerous and truly appalling decisions, and we still haven't really come to terms with them as a country. But I don't think that those decisions had anything to do with nationality. Americans are no better and no worse than the citizens of any other country.