Einstein points out that the humanity of slaves has always been belittled. Maybe it's a psychological switch in the masters brain - enslaving equal creatures would be just evil and no one wants to be evil.

I think pointing out the root cause of the stigmatization of a segment of a society is a very good way to start a social discussion in the modern age.

Unless we as a species are to face our limitations and prejudices with open minds I fear there will be much anguish in the future history of mankind as well.