War is a process of economic (humans are an economic resource too) attrition.

Warfare was originally intended to kill people who were trying to kill you because they thought you were trying kill them first: nowadays it has been discovered that injuring your opponents is more advantageous economically in defeating them, because they have to expend more resources in caring for the injured (of either side if they are genuine humanists) tan just burying the dead so they aren't killed by disease and pestilence.

"Take no prisoners" can make economic sense in desperate situations.