Why is it contrary to military law then?
Military law is VERY strict, for a reason, remember Abu Ghraib? But, in certain cases acts like this should not be punished IF the man that was killed can be proven to be "FUBAR" I guess the soldier should of let the man lay there and bleed to death in pain, or worse try and stop the bleeding only for infection to set in and kill him from fever.
Because sometimes military law is stupid? "Rules are rules" isn't a good defense for a stupid rule.
If the guy and his witnesses are telling the truth, I'd have a hard time convicting him of anything. The guy he put out of his misery had half his head blown of and was going to die anyway- his last minutes (or hours) were likely going to be spent in incredible pain, if he had the ability to process pain with half his brain gone.
If he wanted to be malicious, there's a lot worse things he could have done. Sounds like he choose to make the most compassionate decision he could at the time, and he didn't care about the consequences, because doing the right thing was more important to him.