He had loved Martha more than his men, and as a consequence Lavender was now dead, and this was something he would have to carry like a stone in his stomach for the rest of the war. (The Things They Carried.42)
It's hard to say whether or not we're supposed to think Jimmy Cross is being unreasonable here. On the one hand, it's our automatic response to reassure him and say that just loving Martha couldn't possibly have caused Lavender's death. On the other hand, this is war, and he's the commanding officer. It's very possible that his distraction over Martha caused Lavender to die.
In any other circumstances it might've ended there. But this was Vietnam, where guys carried guns, and Dave Jensen started to worry. (Enemies.2)
We see a situation here that has guilt and blame over a couple of simple things—a stolen jackknife, a broken nose. Simple enough. But, in Vietnam, those kinds of things fester. The presence of guns and the overall tension amplify the guilt and blame and make them so much worse.
Later we heard that Strunk died somewhere over Chu Lai, which seemed to relieve Dave Jensen of an enormous weight. (Friends.13)
Dave Jensen feels guilty for not killing Lee Strunk and putting him out of his misery. It seems as if the soldiers are in a damned-if-you-do, damned-if-you-don't situation: Jensen would probably have felt guilty if he'd killed Strunk as per their original pact, but not killing his friend made him feel guilty, too.
This is why I keep writing war stories: He was a short, slender young man of about twenty. (Ambush.1-2)
While in this book, it's never wise to trust a statement as clear and as truthful as this one seems to be, we'll take it at its word for now. The guilt over killing the young man on the trail is what makes O'Brien write war stories. He gives the young man a history and a wife. He's trying to bring the young man back to life with stories.
Speaking of Courage
"The truth," Norman Bowker would've said, "is I let the guy go." (Speaking of Courage.127)
This is the truth that Bowker can't talk about to anybody: He let Kiowa go. His seven other medals, everything else he's ever done? None of them count to him because he let Kiowa drown in the sewage field. And it wasn't because he wasn't physically able to hold onto him; it was because of the smell. In our deepest, darkest moments, we can all imagine ourselves doing something like this. We pretend we would play the hero, but a secret, shameful part of us knows that maybe, just maybe, we couldn't. And Bowker carries that guilt.
In the Field
"Ten billion places we could've set up last night, the man picks a latrine." (In the Field.28)
Mitchell Sanders blames Jimmy Cross for Kiowa's death. The blame is not entirely without merit, but Sanders doesn't just blame Cross because it's his fault (after all, as Norman Bowker points out, who knew it was a sewage field?). He also blames Cross because he needs someone to blame. He can't accept that Kiowa's death might just be meaningless and sad.
Like Jimmy Cross, the boy was explaining things to an absent judge. It wasn't to defend himself. The boy recognized his own guilt and wanted only to lay out the full causes. (In the Field.47)
O'Brien (we assume that the boy is Tim) so automatically accepts his own guilt in Kiowa's death that he doesn't even try to rationalize it. Instead, he methodically goes over every single reason that he's guilty.
"[…] I felt sort of guilty almost, like if I'd kept my mouth shut none of it would've ever happened. Like it was my fault." […] "Nobody's fault," [Bowker] said. "Everybody's." (In the Field.105).
Even Azar—Azar, people! He kills puppies!—is not immune from feelings of guilt. This is a rare moment of introspection for him. He feels somehow responsible for Kiowa's death simply because he was cracking jokes about the way he died. When he sees the body, the guilt really hits home. Bowker, meanwhile, foreshadows Jimmy Cross's musings (coming up next!) when he points out that the guilt is both everyone's and no one's.
First Lieutenant Jimmy Cross
"When a man died, there had to be blame. Jimmy Cross understood this. You could blame the war… A moment of carelessness or bad judgment or plain stupidity carried consequences that lasted forever." (In the Field.115)
Jimmy Cross (surprise, surprise) blames himself for Kiowa's death. He goes over all the many, many places that blame could be assigned—the war, the rain, God, munitions makers, voters, etc.—and concludes that while the blame is in some ways universal, it's also intensely personal. Jimmy chose to camp in the field despite the warnings of the old Vietnamese women. He is, in fact, to blame.
There were many bodies, real bodies with real faces, but I was young then and I was afraid to look. And now, twenty years later, I'm left with faceless responsibility and faceless grief. (Good Form.7)
Civilians might assume that ignoring the face and body of a man you've killed makes the guilt easier to bear, but O'Brien is saying the opposite. He has to give the dead a face and a past in order to cope with his guilt. It's why he created the young man on the trail to My Khe.