Hmm. As I read the article, it seemed like the author was alternating between suggesting that experience was what turned people that way ("he was the FNG [ed: fucking new guy] that got blown up because he was incompetent, who left the fight before it turned him into one of us."), and suggesting that there is something else at play ("only roughly 20% of combat troops ever get PTSD – when if you think about it, it should affect everyone that ever sees combat."). I read it as him carefully suggesting that the career self-selects for sociopathic tendencies.