I agree, that is an interesting view and it's worth looking at it from that perspective, but I would disagree with the necessarily negative tone of it. That is to say, Rachel's death at such a young age may not have been a happy thing for her, but since we all have to die sometime then going out fighting may be considered satisfying and preferable in her book to trying to adjust to a 'normal' way of life in a world no longer familiar to her. For Ax as well, though it's not clear what his exact fate as an individual is after the encounter with The One, it is probably safe to say that he would prefer to be known for having lived up to his honor and bravery as a prince by confronting the enemy rather than showing cowardice.
If I were to express the sentiment/moral that KA reached through the conclusion, I would remove the negative aspect (That enjoying the fight more brought proportionally greater pain or that war will destroy you if you enjoy fighting it at all) and say that more generally the moral was along the lines of the Greek traditional philosophy that "A Man's [Person's] character is [or determines] His fate" which is seen throughout their mythology and theatrical works.
Actually this is well supported by Rachel's philosophical musings in the previous Rachel book,#37, The Weakness; she had studied Oedipus and other Greek tragedies recently in school and examined her own hubris. Her eventual conclusion was another way to word this: "So it's a trap. An inevitability. You are who you are. Character is plot. Character is destiny." Not necessarily a negative thing, although I suppose 'trap' is a pretty loaded word, removing one's freedom to choose their own fate. But if you assume that one is ultimately, through free will, in charge of determining the kind of person that they are, then the message is simply that who you decide to be determines the nature of your end, or again as the Greeks said a couple thousand years ago (I guess there is nothing completely new under the sun): "A Man's character is His fate."