This is friggin' WAR! The only "moral high ground" is surviving and seeing that your buddies (as many as possible) survive. If popping a cap in this punk-ass terrorist help to insure that this Marine and his comrades come home safe and in one piece, then by all means SHOOT THE BASTARD!
Moral high ground ... sheesh!
...insure [sic] that this Marine and his comrades come home safe and in one piece...
Your reasoning is ridiculous; the conditional you set up can't be the sole determiner for our actions. Sending him back to the states would ensure he comes home safe, but it would be absolutely stupid and wouldn't accomplish the mission. Dropping the moral high ground would also not accomplish the mission, so it's also stupid to do.
The proper approach is to adapt our approach to address the enemy's methods, but not compromise ourselves or our mission in the process.