why are we obligated to act in accordance with the geneva convention?  id think as a former member of the armed forces and a person who's been around long enough to know how things work, this would be obvious.   a treaty or agreement is a contract and works the same way as any contract whether the parties to it are indiduals or nation states. by signing to indicate our agreement, we pledge to obey the terms.

why sign it in the first place?  because it makes practical sense, providing our troops and citizens protection when engaged in combat involving other signatories--even tho that isnt always the way it works out.  should another signatory violate the terms, it loses protection of the conventions and risks prosecution by other signatories thru established procedures.
guerilla fighters who comply with the conventions are due the same protection as signatories and are accorded the same status as signatories. those who dont, arent.

like any other contract, violation by one party does not release any other signatories from their legal obligation.

it hardly seems reasonable or likely the marine youre referring to will be charged by the us military with a criminal act.  just as when a cop discharges his weapon or injures a suspect in the course of doing his duty, the department initiates an investigation--not necessarily in hope of finding a violation, but to determine the facts.  an officer involved in a legitimate or in-policy duty-related shooting should welcome the investigation so as to be offically cleared and to resolve any questions or rumors regarding his fitness to perform his job. same thing with this soldier.

jumping to conclusions, expressing or accepting emotional opinions without benefit of knowing the facts and/or making inflammatory remarks only confuses the issue.  the best way of supporting the marine in question is to avoid getting all wound up based on the limited information provided to the public. at best, youll be upsetting yourself needlessly; at worst, youre making it more difficult to determine the truth. contributing to a rumor campaign can negatively affect his record even tho thats the opposite of your intention.

of course we should.  i dont think that holding an inquiry is obsessing over the convention.  im sure the marine will be cleared because he reacted appropriately considering there were boobytrapped wounded and corpses.  the problem with occupying a sovereign nation in which some of its citizens oppose the occupation is there will be resistance. 

if all the other nations of the world decided the contested election in san diego required them to intervene and invade san diego, some citizens of san diego would very likely continue to resist should the city officially surrender would they not?  in my opinion, theyd be justified to do so.  the occupiers would no doubt feel justified in trying to put down the san diego insurgency. since the invading nations are claiming to act for the ultimate good of the majority of san diegans, they would be obligated to operate according to any rules of war theyd previously agreed to recognize.  the san diego insurgency--not having agreed to any such accord--would be free to fight however they felt necessary with the caveat that they were putting themselves in grave jeopardy and, should they lose, likely to be prosecuted as are street gang members.  

if, during the conflict with san diego's insurgents, the occupiers violated their own commitment, they would jeopardize their own claim to be acting on behalf of the city of san diego and, should they lose the conflict, the commanders might well be put on trial for war crimes.

clearly it's in our interest to adhere to the convention no matter how irresponsible the san diego resistance militia comports itself.