I try not to emphasize the screwups of the troops on the ground in Iraq. I can't imagine what it's like to be in their situation, but to the extent that I can I imagine I'd shoot at anything that moved the wrong way or looked at me funny, and then spend the rest of my life haunted by that.
Whatever personal responsibility any individual soldier has for what they're doing, in the end the blame is on those who put them there and the situation they've created.
The slaughter of the Iraqi policemen and the too little too late official apology for it is just an incredibly bad development. Those signing up to be police are already targets from people opposed to the US occupation.