I'm going to be that guy and ask - why? Wouldn't that be more of a cultural thing? Wouldn't some good PCs not think poorly of slavery? I barely remember a series I read as a teen where the PCs (best term for them) were all about freeing slaves, but they initially came from our world and had our sensibilities.
Chattel slavery and indentured servitude, for purposes other than punishment and/or containment for violent intransigent psychopaths, is non-good because it violates the dignity of the human person and it dehumanizes the slave-owner, since, given human nature, having total control over other humans encourages predation on them.
Further, slavery negates the possibility of choosing between good and evil, the possibility of which allows people to either grow in perfection or fall into corruption, and it is this freedom which characterizes properly human acts.
Obviously, more primitive societies have not had the time to think through the ramifications of their actions, thus their level of culpability varies and must be adjudicated by the game master based on a variety of factors. This is not license to re-define what is good, but rather a distinction between neutral and evil.
The institution of slavery served important functions in primitive cultures. It acted as a prison system, it mobilized low-IQ individuals who would otherwise consume vital resources at a time when mass starvation was a real possibility, it asserted control over fractious individuals in a society which lacked a modern police force, and it served as a system of enculturation for foreign peoples, whether introduced to society by being captured in war, or whatever, thereby preventing them from destabilizing society.
As society develops more sophisticated institutions, piece by piece slavery becomes increasingly antiquated, becoming more destructive than constructive.
Roman society was shockingly violent by modern standards, even during the best of times. Slave revolts were common. During the slave revolt known as the Third Servile War, 50,000 combatants died, not including the 11,000 who were crucified. In our own society, slavery ended in the bloodiest war in American history. The inaccessibility to justice causes despair in a population, and it is this fundamental failure of Ancient culture which lead to the development of Christianity.
As a reminder for translation purposes, the modern theological definition of evil is not that it is a thing unto itself, but that it is the absence of good. Thus the neutral and evil alignments both translate into the real world as evil, the difference being the level of culpability. Those of neutral alignment will commit evil acts when they are provoked into doing so, or where it is pragmatic, and this characterizes the majority of human history. Whereas those of good alignment consciously resist committing evil acts, provocation notwithstanding.
So, slavery is not evil because it does something, it is evil because it lacks something.
It is perfectly natural for slave owners to rape attractive slaves, because that's just human nature. It is perfectly natural for slave owners to brutalize and possibly kill slaves who make them angry. There is simply nothing in slavery that encourages the betterment of the human person.
To think of this in modern legal terms, if you kill someone for having an affair with your wife, a natural response to being provoked, you will normally be charged with second degree murder. That such an act might seem perfectly reasonable in other cultures would not be a viable defense since it is assumed that human rights are universal regardless of whether they are practiced universally.
Nazi Germany and the Soviet Union committed acts of predatory violence, including slavery, that were fundamentally no different than the actions of Ancient and Medieval states, aside from the industrial scale of their acts, made possible by modern industry. After WWII, the world came to a consensus on the nature of human rights. These ideas were not new, having existed for thousands of years, only the global scope of the consensus was new. Which is to say that people did not have a different notion of what was good thousands of years ago -- people still hated injustice, but there was nothing they could do about it; predation was rampant. States such as the Islamic State of Iraq and the Levant, which in the Ancient world would have been considered a stabilizing and civilizing influence, are now universally regarded as a pariahs.