"killer slaves" is a bit dramatic.
I always like to point out that the predecessor to the Western civilization, the ancient Mediterranean, including Greek city states and then later on Rome, was famously built upon slave labor. Even the democratic Athens had a large slave population. However, in a Greek city state like Athens (or the earliest form of Roman legions) the free men -- the slaveowners of the society -- were also the hoplites fighting the wars the city would be involved in. Being a citizen of such a city-state is one the origins of the Western concept of citizenship.
The distinction if you are able to view your community as "us", towards whom you have obligations and duties, not as "them" who rule over you.
Being forced to fight in a war is being 'controlled by something'. In this case, a government. They're government slaves.
Here we are discussing slavery as a concept, which is different.
'Slavery is, in the strictest sense of the term, any system in which principles of property law are applied to people, allowing individuals to own, buy and sell other individuals, as a de jure form of property. A slave is unable to withdraw unilaterally from such an arrangement and works without remuneration. Many scholars now use the term chattel slavery to refer to this specific sense of legalised, de jure slavery. In a broader sense, however, the word slavery may also refer to any situation in which an individual is de facto forced to work against his or her will.'
Note:
'In a broader sense, however, the word slavery may also refer to any situation in which an individual is de facto forced to work against his or her will.'
The broader description isn't a metaphor.