Combat stress
The soldier involved needs psychiatric help. He is just another "casualty of war". Having been shot in the face the day before, he is back out on active duty, non-plussed, stressed beyond normal endurance and performs what he sees as a simple mercy killing. (He also has a bad taste left in his mouth from the "enemy")
I can see how that happens. After all, it IS "war".
Cut the boy some slack, send him home to heal.