Orthodox Christianity is not concerned fundamentally with morality as an end in itself. The vocation of humanity is for deification, participation in the eternal life of the Holy Trinity. Human beings are to become by grace all that God is by nature. A common image for theosis is an iron left in a fire until it takes on the qualities of the fire. It then glows red hot and transmits heat to anything that it touches. Likewise, human beings are called to shine with the light and life of God, to participate fully in the healing and fulfillment that the incarnate Son of God has brought to the world. All are called to embrace and be transformed by the holiness of God, to become saints.
In this light, it is not hard to see why warfare, and any taking of human life, is fraught with spiritual peril. Death comes into the world as the result of sin. Christ has come to conquer death, to raise humanity to the eternal life for which humanity was created. To kill a human being is to do the work of death, to involve oneself in a paradigmatic act of spiritual brokenness and of estrangement from God and neighbour.
Fr Philip Lemasters, Orthodox Perspectives on Peace, War, and Violence