I have spoken recently to a soldier returned from Iraq who specifically had that happen. They received fire and the shooter took up a posture behind a group of children and CONTINUED to fire.
I was asked. What could I have done?
The right answer: You must return fire to protect your own life. You must. To do otherwise is to advocate suicide. You must return fire until the threat is neutralized.
I'm not entirely convinced. Death may be preferable to killing a civilian ("I am an American, fighting in the forces which guard my country and our way of life. I am prepared to give my life in their defense."). But, that's an extreme knife-edge case that, if the defense could show at trial, should result in an acquittal - if the Marines would prosecute this at all. But, as sinky rightly observed, the Marines are acting with such a gravity that it is unlikely that even this occurred.
Hmmm, a moral dilema. I think you're simple answer is the appropriate one for the case given. I wouldn't cap the guy until I had a clear shot though. Just takes a little longer, that's all.