Funnily enough I was thinking about this self same moral question the other day and as Markintime says below about the falklands war...
I recall that whilst no disciplinary action of any kind was brought against the medic who put the Argy out of his misery (and it is well documented how severly injured and in agony the guy was in) apparently his career was 'put on hold' for a long time afterwards.
I guess people who werent there should not be in a position to pass any judgement whatsoever on somone who has to make a quick decision on whether to take a life to end suffering.
If it were me in that boat, then yes I think I would kill a mortally injured enemy to take away his suffering.
Okay, would any one kill a badly wounded comrade.