Apologies if I misunderstood but it seemed as though you were using that as justification for what this marine did. I agree that atrocities do occur in war but that fact in itself shouldn't mean that the same can happen in future wars.
Having never fought in one (I hope I will never have to) I...
I fail to see how he was 'protecting his country from a terrorist' to be honest.
A clear breach of the Geneva Convention which deserves punishment. Or we could always ignore the convention and go the route of the Japanese, Russians and Germans in WW2 and how they treated their prisoners...