Human beings die and kill to prove the reality of the sacred ideal that constitutes the foundation of their society. 

The source of human violence is not biology nor survival (dead men don't pass their genes along). Human  violence, rather, grows out of our spirituality: devotion to an abstract entity that is worshipped by a national group.

The nature of the entity or idea for which human beings die and kill is fungible. Each society possesses a sacred ideal which it worships.

The mechanism is the same, regardless of the nature of the idea or entity: death and memorials erected for those who have died—testify to the reality  of the sacred ideal. Death and memorialization perform a validation function.

The idea of "male aggression" is a smokescreen: denial of the masochistic source of violence. The lie that encourages men to die for their nations; for societies to kill them.

Pretending that a men are heroic, appealing to their desperate wish to prove their "masculinity," nations send their young men to die.