The video is important because it shows the kind of tragedy that is absolutely inevitable in wars likes the ones America has been fighting in Iraq and Afghanistan, but especially in urban Baghdad: where a journalist and a militiaman can appear indistinguishable, where a gunner surrounded by noise and heat high in the sky will fail or choose not to look for complicating details in the scene far below, and where a van taking away a wounded man might be a legitimate target if it were a military vehicle in a conventional war. Those who say that incidents like this have been common in Iraq and Afghanistan are not wrong. The military’s claim that the soldiers followed their rules of engagement is probably not wrong either (though the attempted cover-up invites suspicion). Anyone who sends young troops into war should expect them to kill innocent people by mistake, and to crack jokes about the people they’ve killed. This doesn’t make them war criminals, or even moral monsters. Nor is it the whole truth about them, or about the war. But it’s a truth, and it should be seen.
The WikiLeaks video is not an indictment of the individual soldiers involved -- at least not primarily. Of course those who aren't accustomed to such sentiments are shocked by the callous and sadistic satisfaction those soldiers seem to take in slaughtering those whom they perceive as The Enemy (even when unarmed and crawling on the ground with mortal wounds), but this is what they're taught and trained and told to do. If you take even well-intentioned, young soldiers and stick them in the middle of a dangerous war zone for years and train them to think and act this way, this will inevitably be the result. The video is an indictment of the U.S. government and the war policies it pursues.
All of this is usually kept from us. Unlike those in the Muslim world, who are shown these realities quite frequently by their free press, we don't usually see what is done by us. We stay blissfully insulated from it, so that in those rare instances when we're graphically exposed to it, we can tell ourselves that it's all very unusual and rare.