Maybe you would enjoy reading Martens, Vattel and others if you did not compare their statements with the state of an utopian world without brutality, crime and war; but instead with how war actually was practiced before, e.g. in the 30 Years' War.

To put together rules of conduct which have been agreed on by some belligerents in the past and codifying them as an example to follow for others the future is a meritious work. It puts some kind of moral pressure to behave in a not too uncivilised manner, and thus over the course of the centuries helps to turn the cruel war into a cruel war which are little bit less cruel.