I think PTSD, and the troubles war veterans have, says a lot about our species; namely, that it is totally abnormal to take another human life. It is unnatural, not something we are wired to do.
All militaries train soldiers mentally as well as physically and technically, and part of that mental preparedness is the ability to take a life. We do this by any number of methods, the primary being to de-humanize the enemy. In WW2, the Japanese were portrayed as buck-toothed vermin. Germans looked at Russians as sub-human, implying "It's OK, kill them. They are not human." Religious extremists use the "Our God says it's OK" twisted logic.
All very sad, and against human sensibilities. Which is why war comes back to haunt participants.