There is a powerful natural taboo, even among soldiers, to the taking of human life. But the taboo has been breaking down both in battle and in society, with obvious results.
Studies of wars prior to World War II indicate that as many as 85% of ordinary soldiers must have done their best not to kill. During World War II the rate at which soldiers actually fired their guns was about 20%. During the Vietnam War it was more than 90%. This increase was brought about by training: killing is turned into a conditioned reflex; the "enemy" is demonized by political or racial propaganda; group pressure is intensified. The American soldier in Vietnam was desensitized and conditioned to overcome the normal resistance to killing.
Humans disavow their essential humanity when they kill.
All carnivores and omnivores and most herbivores kill in order to survive. Some kill animals and others plants. Only the majority of plants are free from the need of direct killing, although few would survive without the death and decomposition of living organisms.