Let’s get the difficult stuff out of the way first.
The world is a very unfair, morally gray place. You can shoot a man in war to defend your homeland and he’ll die crying and clutching pictures of his family. We’ll be disturbed on a fundamental level when we’re forced to acknowledge that our enemies are good people. For soldiers, this frequently results in PTSD–no, it’s not just from the explosions and gunshots and seeing their friends die. PTSD can come from killing people, too.