The line you ask about from "The Things They Carried" indirectly suggests the idea that the fighting of a war is absurd and uncontrolled and chaotic and that war is not an idealistic endeavor.

In the setting of the story, the men are not fighting for ideals.  They're just doing what they're ordered to and trying to survive.  Specifically, the line you ask about suggests that soldiers' pride and machismo will not allow them to do anything that would embarrass or humiliate them.  They fight not for a noble cause, but to avoid humiliation. 

But this is not negative or shallow.  These men are trapped.  They are in an impossible situation and maybe the only thing they can accomplish is to avoid shame and guilt. 

Of course, shame and guilt can result from fighting as well as not fighting--depending on the circumstances. 

I don't know if explicate is the best word, but the line demonstrates the situationally ironic world the soldiers exist in.  Ideals do not motivate them, but only the need to accept themselves and be accepted by others. 

It has been shown over and over again in numerous studies (read the first chapter of The Face of Battle by Keegan if you want a good description of it) that men are afraid to pull the trigger and actually shoot at other human beings but are also afraid to be seen as a coward.  Very few people want to be brave or try to be a hero, but no one wants to be seen as a coward.  So O'Brien's description of why men killed and were killed is linked directly to this idea of shooting back for fear of being seen as weak and also doing things known to be stupid and dangerous (and dying for doing so) out of the fear of being seen a coward.  In some ways it is also to contrast the idea that many people see those actions in battle as coming from an actual desire to kill or desire to be brave.  Generally, it is quite the opposite as O'Brien so eloquently shows.