I think this might give some answers:
World War II casualties - Wikipedia, the free encyclopedia
In a somewhat connected statistic, i read that around 80% of military personnel active during WWII never fired their weapon in anger.
For frontline troops this number was around 25%. Thats 1 in 4 who refused, or was unable, to fire at another human being. Due to new training methods, this number has changed in later wars and is assumed to be around 5% now.