The pattern and concentration of damage is around the doorway, with the doorjambs getting hit the worst. Some of the damage is 2-3 times the height of the men standing there in the picture (Hoplites's post #48) which makes it appear those shooting couldn't aim correctly and calmly. So, it appears they were concerned over others inside the doorway taking cover there and shooting back. The gunfire could be aimed in the general direction, but not precisely. If people were shooting at unarmed POWs they could do so without incredibly tearing up the buildings face.
I guess it's the garage area where allegedly 1,000 Muslims were crammed before they were shot, so the story goes. But could 1,000 men fit? Why would the gunfire be concentrated around the doorway off to the side of this, and become much, much more dilute around the garage area if that's where the target was?
Great points.