Killing people doesn’t come easy to the vast majority of people, that’s why many armies train their soldiers so hard, demonizing the enemy and brutalising their own people.
Some soldiers have to adopt the psyche of a brute in order to kill another human being. This in itself de-humanizes both the soldier and his victim. When it’s all over, most of the time, soldiers are rendered traumatised by their experiences.
A friend of mine was concussed by a mortar bomb. As he came around he saw one of the enemy stooping over him. As a reflex action, he immdiately stabbed the enemy with his combat knife. As it happened, the enemy in question was stooping over him to give him water to drink. My friend wasn’t traumatised by this, he just accepts that war is nasty.