It's probably worth noting that many ancient societies had mystical/religious rituals to separate a soldier's identity in combat from his identity at home. It wouldn't have solved every problem, of course, and it probably came with a host of its own, but I can't help wondering if something similar would help our returning vets.
On the other end of the scale, you have societies like the Spartans, who (so I've heard) approached war the way a laborer approached work...messy and often unpleasant, but necessary. I've heard that they even used farming terms to describe combat, to help reduce the emotional impact...the front line of their phalanx engaging an enemy was called "reaping", for example, particularly when they got close enough that they had to rely on their short swords rather than spears.