Disclaimer: If this turns out to have been an actual revenge massacre, then I hope all guilty rot in hell. I add this because apparently the overexposure to patchouli renders some unable to understand my position. I will shorten this to: If Massacre, Guilty Hang (IMGH) for convenience.
The following is my view of how the determination will be made as to whether this was a tragedy or a travesty. It could serve as a simple primer about when shooting civilians is sadly acceptable and when it is criminal. I will not discuss the possibility of a massacre because then all of these calculations are void and IMGH. I lay this out as it will likely form the basis for a defense to any charges that may come down.
These are some of the implications of shooting a non-combatant during immediate action and house-clearing operations.
Disclosure: I have never been to Iraq and have no direct knowledge of this incident or anyone involved. I do have extensive experience training teams in hostage rescue and close quarters battle (CQB) and have been on countless training and multiple live operations kicking doors in.
There is one and only one relevant standard for this whole incident and that is the reasonable belief that your life or the life of others is in danger. If that is reasonably believed then deadly force may be employed, absent that belief it may not. There are other factors that would impact this, like rules of engagement, but in this case the Marines ROE obviously included the right to return fire. The question is was there any, and if not why did they fire? If the Marines killed people without reasonable belief they were in danger, then they committed murder. If they had a reasonable belief in their danger, they did not.