As a part of a written history assignment about slaves in the ancient world, I analysed a series of letters from Seneca to Lucilius. (It’s unclear though if the letters were ever sent, but we know that they were written with the intention of being publicized)
Seneca was a member of the philosophical school of stoicism that sought to free people from material wealth, and make every man free (like a precursor to liberalism).
In these letters, Seneca celebrates Lucilius’ friendly, and civil relationship with his slaves. He then proceeds to discuss whether you should use the word “slave”, and not rather “human”, or “roommate” (I promise you that the word makes sense in the context of the letters), or “friend”. He ends up using “co-slave” as the slave owners are no better than the slaves themselves (he elaborates on this point throughout the letters).
Seneca expresses empathy towards the living conditions of the slaves; that they are not allowed to “break the silence”, nor make natural, and “random” sounds like hiccups and sneezing.
He criticizes the, at the time popular, idiom that “you have as many enemies in your house as you have slaves”, and says that slaves need not be enemies, but become so from the way we treat them; the tendency to forget that they are human, and therefore being able to justify treating them as “talking tools”.
Looking at the viewpoint of Seneca expressed in these letters, we see a very modern view of human rights which was quite unusual for his time.
Coupling this with the fact that humans tend to be more cooperative in a positive environment; that is an environment where they are not being treated as “talking tools”, would suggest that more than a few slave owners treated their slaves in a somewhat kind manner.
Furthermore, there are a few historic accounts of domestic slaves (slaves working in the master’s private home) being entrusted with taking care of the children along with other sensitive things which you wouldn’t be able to do if you didn’t have a somewhat civil relationship with the slave.
That is not to say that all slaves were treated in a humane way. Archeological evidence shows numerous slaves being worked to death in harsh conditions while being malnutritioned, and subjected to bodily stress.
Note:
In an attempt to write more regularly, I'll conduct an experiment where I cross post some answers from Quora on Steemit. I'll in the coming time try to figure out to what extent this practice should be done.