The reaction of USA senators illustrates this: while they are dismayed at the pictures of torture, they think that the beheading is worse, and somewhat justifies things. A beheading is not very nice but it is fairly swift (disclaimer: I have not viewed the video). It does not involve humiliation, which I think is what really gets the Muslim side's goat. So it is comparable to, say, a soldier of the USA shooting an Iraqi citizen -- which has happened quite a lot.
This attitude had been in place for some time on the USA side. What has changed is that the Muslim side is now starting to adopt it also, and may stop being so discriminating in their targets. Large scale massacre of westerners, which the Muslim side has previously refraned from, is now possible. Large scale massacres are relatively easy to carry out, and have been all along. WMD has always been irrelevant. The simplest method that springs to mind is to compromise the water supplies of some large cities. Targeted killing takes finess: choosing a target that sends the right message, scripting an impressive drama to make sure people get that message. It is essentially a violent form of negotiation: kill some accountants to say we don't like your economic system, kill some tourists in Bali to say stop eroding to dignity of Muslims, stop trying to seduce them with your wicked ways, kill some police in Iraq to say don't accept the occupation. This negotiation is now ending, and we are heading for total war.
It is a war of Morality. Two sides are shouting loudly. One for a strict code of morality, self sacrifice, and rigid hierarchy to save us from the taint of evil that infects all people. Believe in the word of the book, for to consider alternatives is to invite chaos. The second side is for atomized individuality, hedonism, self-esteem: do as you will, but neither help nor harm others. There is no truth but what you believe, because you are alone in the universe.
What i think: Muslims extremists, your cause has some justice, though you are wrong about many things you basically mean well and want to help as many people as possible find salvation. Your methods however are futile and destructive, and you are going to die. People of the USA, your cause is not just, you are acting solely from pride, grow up. I expect growing up to be painful, it did not have to be painful, but now it is going to be painful and you deserve it. Your dominance of the world is over, you're broke.
There is only one way to win this war, and that's to say "our civilization is better than yours, come join us." Both the Muslims and the USA have now no chance of saying this and meaning it. Therefore, if there is a winner, it will be neither the Muslims nor the USA.