I'm not saying this makes it OK, or that the people who carried out the apparent massacre shouldn't be punished. In a large city, there's inevitably going to be some crime, but that doesn't mean you don't punish the criminals. In war, there are always atrocities, but that doesn't excuse the people who commit them. That these particular Marines reacted to the bombing death of a comrade in such a horrible way wasn't inevitable, but it was inevitable that someone would do something horrendous under the pressures produced by a war of this magnitude (Abu Ghraib, anyone?).
But the fact that horrific things happen in war is relevant--to the decision to go to war in the first place. George Bush didn't intend for 24 civilians to be murdered. He didn't particularly want to kill tens of thousands of Iraqi civilians over the course of three years. He didn't want Iraqis to have the misery caused by their rotten water and power supplies. But all of this has happened because he did want to invade Iraq. And if the details weren't predictable--no one could have said three years ago that there would be a massacre just like the one that seems to have happened in Haditha--the fact that lots of people would end up dead, orphaned, permanently injured, or miserable was perfectly predictable.
That's why, from Catholic just war doctrine up through contemporary international law, the morals and laws applicable to war have said that you can't attack someone without a very, very good reason: traditionally, only self-defense or the defense of another country against aggression (e.g., the first Gulf War on behalf of Kuwait).
Bush's eagerness to go to war, and the deception and willful blindness he used to get us there, are morally reprehensible. He didn't kill anyone in Haditha, and he may well be horrified by what happened there; but he knowingly created the conditions in which here was a significant risk of such things happening. And he gave no evidence, then or now, that risks like this played any role in his decision whether to invade. He's not morally responsible for killing these particular people, but he is morally responsible for recklessly putting millions at risk without giving their lives any moral weight at all.
Sometimes, I wish we had a president who acted like a Christian instead of loudly announcing that he is one.