I would like to think that if i needed to kill, I could do it.
I would hate to be the poor bastard who sat in a corner crying while someone killed their family.
On another note, using the channel 4 documentary as an example, you can see how technology is making it easier to kill. The knife is the hardest weapon to use in war psychologically. Not because you are killing, but because you have physical contact with the person you are killing. A pilot at 20,000 feet dropping a bomb on a tank with 5 lads in it is in a much better position. He "knocked out a tank". Whereas the grunt has just knifed a man who was likely screaming for his children.
Technology is constantly putting the enemy at a distance. A thrown stone, a spear, javelin, arrows, rifles, cannons, artillery, bombs. All have progressively distanced the killer from the victim.
It is a fact that modern armies are becoming much better at making young men kill. Wargames play a huge part. If you spend weeks in a mock urban environment firing paintballs or lasers at other soldiers, your reflex action in a real battle will be the same, and the moral argument disappears, and war becomes a game. Until after. Even then you will find that the American and British armies have some of the best psychological personnel in the world. Because you need soldiers not only to recouperate from killing, but you also need them to kill again...