Why does O'Brien insist that war stories are not moral, and why does he try to reconsrtuct what Lemon must have experienced the moment of his...

In The Things They Carried, O'Brien
gives the recipe for "How to Tell a True War Story" by saying the stomach should believe
it.  It must pack an emotional wallup.  If it doesn't, it comes across as manufactured,
invented, moralistic, political.  It becomes a lie by becoming something else--an
abstraction, a polemic, a sermon--not a story.  A war story is a love story.  It brings
back loving memories.  It resurrects friendly
ghosts.

O'Brien's thesis for the book
is:

I want you
to feel what I felt. I want you to know why story-truth is truer sometimes than
happening-truth (O'Brien
179).

So, O'Brien says that
true war stories need to be light on  facts (logos) and morality
(ethos).  Instead, they must be visceral and brutal and gory and
full of cussing (pathos).  That's the way soldiers talk.  That's
their style.  And it's been this way since Trojan war
mythology.

First of all, war is absurd.  It does not
operate according to ethical, moral, or religious rules.  Bullets are indiscriminate.
 During war, a soldier or storyteller does not have time to moralize.  Morality and war
don't mix.

Remember woman at the reading?  She wanted a
moral or a lesson or some kind of redeeming meaning by the end of the story he read.
 She didn't understand that the story itself is the lesson.  She didn't understand that
it was a love story.

A good story doesn't preach: it's
morals are implicit, if there are any at all.  To the “dumb cooze” audience reading
O'Brien's novel for realistic depictions of war and conventional storytelling his thesis
is a betrayal, not a paradox; O'Brien, to these readers, comes across as weak,
dishonest, one so full of regret that he rejects the traditionally masculine-defined
rules of engagement in both war (“kill or be killed”) and storytelling (unreliable
narrator).

Lemon's death becomes a funny and beautiful
thing.  He reconstructs the memory by draining the focus on death from it.  He drains
the horror from it.  If a soldier and storyteller can do that, then he can open up new
perspectives that are, ironically, both beautiful and funny.