OK, so you were taught this. I fail to see where the moral outrage comes from though. Killing a terrorist prisoner—which in any case he was not convicted of, a point you didn’t address—doesn’t evoke in me any kind of emotional response whatsoever. And why should it?
The reason that many service members don’t like the Gallagher pardon is that they survived combat tours without murdering any prisoners and this clown gets a pardon while they get nothing for following the rules. It is a great insult to warriors.