Why does killing people not bother me?
Well, beyond my very flexible sense of morality, I've been trained and conditioned to do it. I can view it from a utilitarian point of view. For control of an area sometimes strong arm intimidation is required. Even from a socio economics point of view with population on the rise, limited resources... Or even from a naturalist view, we are meant to live and die. It is human nature, If it was not our nature we wouldn't do it... Kill that is.
Many people are taught the sins of murder and conditioned to believe hell awaits those that commit it. But I don't believe in god. I believe in my brothers who I fight with. For my family who I fight for and myself. I protect the things that protect and care for me. This is the clan mentality our species has lived on sense the beginning.
I don't now what narcissism would have to do with the military. They do breed a sense of pride in you, but they emphasis the importance of the unit over the individual.