This may be wrong of me, but even if happened as some have alleged, I'm not going to criticize their actions. It's possible for them to have a room full of suspected terrorists and/or family members in a submissive position, when one of them reaches for something. Maybe a weapon, maybe a dog. Two seconds later, they're all dead.
That was not a massacre. It was an unfortunate incident that had a logical cause.
I bet the marines went into the house and demanded, "Where are the men that did this?" The women said, "No men here, we love America." Then the marines found the men hiding in some trap door or something. The guys in the taxi were probably first rate insurgents. I bet the marines found something on one of these "students" that proved they were involved- maybe a videocamera.