John Calvin Commentary Deuteronomy 17:7

"The hand of the witnesses shall be first upon him to put him to death, and afterward the hand of all the people. So thou shalt put away the evil from the midst of thee." — Deuteronomy 17:7 (ASV)

The hands of the witnesses shall be first. It was not without reason that God ordained that criminals be put to death by the hand of those by whose testimony they were condemned. The ancient people did not employ public executioners, so that there would be more solemnity, modesty, and reverence in the infliction of punishments.

He specifically assigned this office to the witnesses, because the tongue of many is too hasty—not to say worse of it—so that they do not hesitate to stab people verbally when they would not dare to lay a finger upon them. This, then, was an excellent remedy for discouraging frivolous accusations: not to admit the testimony of anyone whose hand was not prepared to carry out the sentence.

Stoning was indeed a grievous and horrible kind of punishment; but it is probable that God chose it because it required the involvement of many hands. If hanging had not been in use, God’s command that the corpse of a man who had been hanged should be taken down from the tree before sunset (Deuteronomy 21:23) would have been in vain. There were, therefore, other kinds of capital punishment.

But when the land was to be purged, as by a propitiation, through the death of the sinner, he was to be stoned by the hands of all the people. This was because it would have been cruel for him to be killed by a lingering death, which would have been the case if they had stoned him one after another. The reason the people were commanded to cast the stones in unison was so that they might demonstrate their zeal and show their great indignation that God’s worship had been violated.