Shooting to kill - how many men can do this?

We ask soldiers to shoot people in combat, but how many men find this easy to do? What proportion can pull the trigger? Modern techniques of training have enormously increased soldiers' willingness to kill, but the consequences of this success are yet to be fully understood. A man who is not a natural born killer may have a tough time dealing with the fact that his training led him to do things that his natural reluctance would have prevented.