If the guy was an enemy non-commissioned officer as indicated, he probably had value as an intelligence source no matter how badly he was wounded. He was given the easy way out of revealing what he knew.
Not to mention that the guy who gave him a loaded pistol placed himself and the guy with him in mortal danger!
That is
THIS old soldier's viewpoint. I would have done every thing I could to keep him alive until he could be interrogated by someone who could communicate with him.
What might be learned could easily have saved some American fighter's lives. I would have brought court martial charges against the guy who gave him a pistol for "aiding and abetting the enemy."
Uniform Code of Military Justice (UCMJ)
ART. 104. AIDING THE ENEMY
Any person who--
(1) aids, or attempts to aid, the enemy with arms, ammunition, supplies, money, or other things; or
(2) without proper authority, knowingly harbors or protects or gives intelligence to or communicates or corresponds with or holds any intercourse with the enemy, either directly or indirectly;
shall suffer death or such other punishment as a court-martial or military commission may direct
(
Get me started and see if I can stifle how I REALLY feel about such behavior!) 
<RANT MODE OFF>
What was done MIGHT be mercy... but it was definitely STUPIDITY.