Soldiers do have trouble killing on the battlefield; have a read about "operant conditioning", a psychological technique used in modern training to overcome it.
General Marshall discovered after WW2 that only 15% of front-line soldiers even fired their weapon at the enemy. Even soldiers don't like to kill enemy troops in the middle of a battle!
Well, first, his claim was 25%, and while my understanding is that it did end up leading to changes in training, you should at least acknowledge the large disputes on how he reached that number (e.g. based almost entirely on interviews available to him opportunistically, no documented methodology, etc).
http://www.amazon.co.uk/On-Killing-Psychological-Learning-So...
General Marshall discovered after WW2 that only 15% of front-line soldiers even fired their weapon at the enemy. Even soldiers don't like to kill enemy troops in the middle of a battle!