“So powerful is the compulsion of the law, that even if a man slays one who is his own chattel [i.e., his slave] and who has none to avenge him, his fear of the ordinances of god and of man causes him to purify himself and withhold himself from those places prescribed by law, in the hope that by so doing he will best avoid disaster.” (Antiphon: On the Choreutes, c. 430 BCE)

Here are some good primary source documents on the subject. http://legacy.fordham.edu/Halsall/ancient/asbook09.asp#Slavery