The personal risk element adds adrenaline. I would expect this enables the typical human to cope better with traumatic events. Also, my personal experience is that I was always too busy to be scared whilst flying.
Another element is the utility of the killings - is there a subconscious effect from the effectiveness, and for that matter the morality, of what is being carried out? I do not wish to debate the absolute morality and effectiveness, but those perceived by the operators, and those perceived by the public at large. Vietnam probably has some lessons here.
What You Have Is Probably Worse Than PTSD
I should think compartmentalisation as a means of handling the stress is somewhat difficult when you blow up an imam and his 3 kids, then see a family group dressed the same in the cereals aisle at Sainsbury's 3 hours later.
The PTSD doesn't surprise me in the least, and I have every sympathy for the operators. The politicians have started stupid wars, penny-pinched the Armed Forces to set up this semi-civilianised form of warfare with all its incongruities, and then lost the wars. It's a wonder everyone isn't crazy.