Charles Ellicott Commentary Exodus 21:20

"And if a man smite his servant, or his maid, with a rod, and he die under his hand; he shall surely be punished." — Exodus 21:20 (ASV)

And if a man smite his servant. —The homicide previously considered has been that of freemen; but the Mosaic Law was not content to stop at this point. Unlike most other codes, it proceeded to forbid the homicide of slaves. Until then, throughout the East, and also in many parts of the West, slaves had been regarded as so absolutely their master’s property that he was entitled to do as he pleased with them. Now, for the first time—so far as we know—was the life of the slave protected.

The exact extent of the protection is uncertain. According to the Talmud, the master who killed his slave was put to death; according to some modern Jews, as Kalisch, he had merely to pay a fine. In any case, the killing was an offense of which the law took cognizance. Later on, it appears that even assaults on slaves, if they reached a certain intensity, were unlawful and involved the slave’s compulsory emancipation (Exodus 21:26–27).

With a rod. —The usual instrument of punishment. It would follow, as a matter of course, that if a more dangerous implement was used, the master was punished with equal or greater severity.