This kind of stuff must happen a lot more than anyone ever gets to find out about.
Some perfectly rational people crack under even mild levels of stress, so putting them in a warzone will always lead to atrocities.
It's a difficult one for me really. I don't disagree with the punishment at all but I think it's really hard to accurately judge the individual. Say if they've been there for a long period of time, seen colleagues shot, dead bodies paraded around. Subject to constant stress and fear of death...then they're presented with a prisoner/enemy/problem. Expecting morals to kick in over survival instincts/hatred/being fecking nuts, isn't always going to be realistic.
It doesn't make the action of killing a prisoner any less wrong, which is why it's difficult for me to have a strong view either way on it. Wars are shite, basically.