Grossman begins with the assumption that human beings have an innate resistance to killing members of their own species. He supports this assumption via references to studies documenting the low ratio of fire in World War II and the disparity between the destructive power of weaponry and the numbers killed in previous wars. The military has learned how to train and condition soldiers to overcome this innate resistance. There is, however, a high psychological cost in overcoming this innate resistance to kill: For 98% of soldiers who kill in combat, there are serious psychological and emotional effects. Grossman argues that these psychological and emotional effects need to be acknowledged and addressed.

Several factors contribute to psychological casualties on the battlefield and after. Aside from exhaustion, all the factors relate in some way to the burden of killing. Grossman explains that the fear that contributes to psychological casualties is not the fear of death or injury. Rather, combat soldiers fear their own inability to kill and worry that they will let their comrades down or, alternatively, they fear that they will kill another human being. The carnage of the battlefield overwhelms combat soldiers with a sense of guilt precisely because they feel a responsibility for it.