To draw a distinction between justified and unjustified violence it is necessary to make a distinction between two periods or forms of living, war-time life and peace-time life. The justified acts of violence are those that will be likely to lead back to a period of peace, and at the same time keep the one doing the violent acts capable of returning to and practicing a peaceful life. The unjustified acts of violence are those that will likely destroy the capacity of the one responsible to return to the practice of peaceful life.
Deliberate killing of children is likely to destroy the killer's later chance of enjoying peaceful, regular, secure economic relations, friendships and loves in his own life, especially in the killer's relations with and education of children.
Children enjoy a special status because, excluded from the class of soldiers, they are identified completely with peace-time life. We know children can be made into soldiers, but we also know that when this happens they will change so much, lose what is characteristic of childhood, that in a real sense they cease being children. If you look ahead to a time war will be over, you cannot intentionally choose to kill children, as you know children live with you under an assumption of security and protection, rely on you being capable of offering to them your love without qualification.
It is important to keep our attention on the ability we have to love, and to look carefully at this claim that we can lose the ability to love, with disastrous effects on the society made by people who cannot love each other. We ought not look for some quality essential to an idea of civilization that certain acts of violence will destroy. We do not have a definition of civilization that is sufficiently agreed upon and clear to allow us to succeed in that argument.
Those who practice terror in fact usually live in a condition of despair, feeling that they have nothing to lose. Those who are not in despair can see that those who practice terror are wrong in their belief they have something to gain, because despair does not end if you form for yourself a character that is fit for no better human condition than despair, and wrong in their belief they have nothing to lose, for no matter how far we fall it is always possible to lose more of our humanity.
When we wage war we know we are acting as lesser human beings, but believe it is better to be lesser now, and then more later, than to be dead and so no human being at all. We all are familiar from our private lives with transitions of this kind, losing ourselves and regaining ourselves. Though we forgive ourselves our failures, we do not knowingly destroy ourselves in the hope of saving ourselves. Or rather we do sometimes, but we know this is madness.