Mind if I join this high voltage discussion?
What I picked up from this chapter is the Samurai Code. It's not too different from the Western Chivalry Code. It still exists to this day. One prime example would be the Geneva Convention...
The MC was doing his part to fit the character the author wanted him to be. Since the MC is only 16 and not some reincarnated 40 or 80 year old person, I believe that's a pretty good assessment of his behavior.
Another thing is that the other guy was from the same home country and was just doing his job to protect his superior. Who knows, but perhaps the MC was think ahead. I'm sure he's hoping that by sparing their lives, he won't be looked upon as some evil monster but just someone who just wanted to go home.