For an idea of why this is an issue, Google "confessions of a drone warrior". RPAS pilots get to know their targets in a way unthinkable before; watch their lives with their families and children for days or weeks, watch in close-up the kill, survivors writhing in agony, first aid attempts then grieving relatives collecting body parts. Then home to tea with the missus. It's 100% understandable that there are likely to be issues with PTSD.