I have a couple buddies that were military sniper , one Marine and one Soviet Spez-Naz , both say that the field craft is increadibly hard and it something that you have to be born to do. The russian fellow said that one of the hardest things he had to do was make a creep accross a section of open type brush land with a serious case of the squirts , he said the hardest part was laying flat on you stomach and crapping pants.
Anyway both guys have seen active combat and both say that they have made shots on human targets and neither have any regrets. both guys work for a Private security company in Iraq right now.
I do have a close friend that was a sniper in the NG and is now a SWAT sniper , he to hase made shotson human targets several in combat and once in LE work the latest one being in LE , that one realy bothers him. Shot was about 50-60yds at night through the back window glass of a truck.What bothers him is the mess it made and the trauma that the wacko's wife and child had to see as they were both in the truck.
So I would say that the mental aspect has a majority to do with it