Weapons are bad in general. The military, however, takes a practical view rather than a moral one, and will not limit themselves based on what is good and bad, but will arm themselves based on what is effective. This is why we can't have nice things.
They may be bad in some game theory way, I don't know the argument, but large groups of humans are very much not rational actors. The tendency is to oppose the enemy rather than to maximize gains or minimize losses.
