I don't anything to add, but I think James Fallows takes the right approach on how to react to atrocities like this:
There will be lot of those "real questions" to consider, from rules of engagement to the apparent cover up of the footage. But the threshold point I meant to start with is this: The very high likelihood of such "tragedies" occurring is a very strong reason not to get into wars of this sort.
By "of this sort" I mean: twilight-zone urban warfare, not to mention "discretionary" or "preventive" wars, and situations in which a heavily armed-and-amored occupying force of foreigners tries uneasily to mix with a population overwhelmingly of a different race and religion and language. For their own survival, the occupiers need to be hyper-suspicious and ever alert -- even though today's prevalent Counter Insurgency doctrine ("COIN") warns of the self-defeating consequences of behaving this way. (Indeed, a mounting debate about the COIN approach in Afghanistan is whether the effort not to seem distant from the local population is exposing US soldiers to too much risk.) It is a situation with enormous potential for miscalculation, misunderstanding, and tragedy. And therefore one to avoid if you have any choice at all.