1) What are our moral obligations as leaders to our soldiers?
There are probably tons, but here are some thoughts off the top of my head:
a. Train them to be proficient in their wartime tasks, so they have the best chance of accomplishing their mission and returning home alive.
b. Respect them as persons--this includes things such as recognizing their family obligations and not humiliating them and looking out for their long-term welfare (encourage GI Bill, financial counseling, etc). A part of respecting our soldiers as people is ensuring that they can integrate their soldierly tasks with their human ones--that they can make sense of the death they see and deal. If we recruit people, train them to kill, order them to kill, and then not help them make sense of killing so they can go on with their lives, then we're using them as mere soldiers, not treating them as soldiers who are people--before, during, and after their service.
2) How do leaders lessen a soldier's psychological impact of killing?
a. Preparation is HUGE. According to killologist Dave Grossman, one of the three critical components of developing PTSD is surprise--facing an unexpected trauma. As leaders, we can help by talking with our soldiers about how upsetting killing can be. It shouldn't come as a shock to them--real war ain't a range.
b. We can also let them know what we as leaders care about fighting morally. They may not have the education or maturity to understand it all, but they should have the confidence that someone they trust does. Then we just have to walk the talk.
c. Third, after a fight we can help by talking about it, and encouraging our soldiers to do the same. Most PTSD doesn't begin showing until months or years after the trauma, when Soldiers have the time to reflect and think about things. If we as leaders recognize this, then we'll be in a position to be there for our troops (including those who fought with other units on previous tours), and we'll know that killing another human being isn't something that most people "get over" quickly or easily.
I hope this helps. Let me know how it goes.