re: just suppose
If I was a bank guard, and Patti Hearst had held up the place with an automatic weapon, I would have shot her, too.
If a retarded boy was heading for a crowd of people with a remote-controlled bomb on him, I'd shoot him too. I most definitely would not enjoy it. But innocent dupes were not the people this Marine was talking about, remember?
BTW, don't ever join the military. Someone would kill you while you were still working out the variables. Many of our enemies have no scruples whatsoever. Besides, I think you're more valuable where you are.