The way I envision Zako girls is as conscript troops.
An evil organization rules a territory and takes over others by force. Peasants and civilians are forced to join the ranks, or die.
They are subject to ruthless discipline and harsh punishments if they don't blindly obey orders.
The general idea is the army (of men, I know!) in the film "The Postman" with Kevin Costner.
The force is held together through fear, with death as the only punishment for infractions.
Officers are indeed evil, but the Zako are not: they cannot refuse to fight, nor retreat.
During battles, the "heroes" have no choice but to slaughter them, as many as possibile. They'll have to fight again those whose lives they'd spare!
On the other hand, WE are allowed to feel sad and pity for the massacre of innocent girls, whose only 2 choices are fall by the hand of the heroes or their officers'!
As for the evil commanders, their consideration for their troops (and the supplied equipment) must be, quoting King Edward in Gibson's "Braveheart":
Weapons cost money. Use up the Zako. The dead cost nothing.
