It must be said that the Stoics were cowardly when it came to slavery. Marcus Aurelius, who believed that we were all part of a common whole, that we were all equal before life and death, who so admired a former slave like Epictetus, who writes at one point about why it would be wrong to have sex with a slave, doesn’t see a problem with owning a person. He had the power to eliminate slavery in the empire, but he just couldn’t do it.
Seneca is an even bigger hypocrite. He writes over and over again about the importance of freedom and kindness and fairness, yet how many slaves did he own? Too many to count. He writes about slavery often in his letters, and you can just feel that as wrong as he knows it is, he can’t come out and question the institution that defined Roman life. He even knows he’s being hypocritical and in Letter XLVII more or less admits it. All he can say is: “But this is the kernel of my advice: Treat your inferiors as you would be treated by your betters.”
Perhaps part of the reason that many Stoics had so much trouble with slavery is that as much power as the Romans had over their slaves, there was someone who had that much power over them. The emperor (indeed Marcus for the entirety of his reign) could throw someone in chains, could kill them, could take their possessions or steal the fruit of their labors. This often happened with capricious and devastating cruelty. Selfishly, stupidly, the lesson they took from this was: If someone can do it to me, why can’t I do it to someone else?
They should have really listened to what Seneca was saying, to that timeless and universal idea we see in countless religions and philosophies and now call the Golden Rule. How would you want to be treated by people with power over you? Now why on earth would you treat people you have power over differently than that?