John Gill 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 or his maid with a rod
A Canaanitish servant or maid, as the Targum of Jonathan, and so Jarchi; and that only with a rod for the correction of them, and not with a sword or any such destroying weapon, which would seem as though he intended to kill, yet nevertheless:

and he die under his hand ;
immediately, while he is smiting or beating him or her, on the same day, as the above Targum interprets it:

he shall be surely punished ;
or condemned to the punishment of being slain with the sword, as the said Targum and Jarchi explain it: this law was made to deter masters from using severity and cruelty towards their servants.