It's a case of double standards here, the so called leader of the Taliban is killed by a drone attack, Bin Laden is tracked down and shot, and it is a re-joyful matter.
But here we have three soldiers who have gone through what some of us could not possibly imagine, and have seen their mates slaughtered, dismembered and their body parts hanging from trees, they then come across a seriously wounded enemy, and dispatch him on his way, nothing that would not have expected happening to themselves, if the tables had been reversed.
Not condoning what they did however, but going part of the way to understanding their feeling at that particular time, remembering that horrible things happen during a war.