John Calvin Commentary Deuteronomy 13:9

"but thou shalt surely kill him; thy hand shall be first upon him to put him to death, and afterwards the hand of all the people." — Deuteronomy 13:9 (ASV)

But thou shalt surely kill him. He did not intend for everyone to privately execute vengeance without a public trial; instead, he referred to the common practice where the witnesses would throw the first stone at condemned criminals, as we will see elsewhere.

For it was an admirable provision that God intended for those who had denounced the crime to be the executors of its punishment, so that they would be more cautious and moderate in giving their testimony.

The reason, which is added at the end, because he hath sought to thrust thee away from the Lord thy God, who brought thee out, etc., again magnifies the seriousness of the crime on account of its ingratitude. This ingratitude was detestable in proportion to the invaluable blessing of their deliverance. It was an act of extreme wickedness to rebel against God after they had known Him, but it was even more extreme to undervalue their Deliverer.

Finally, the advantage and benefit of this severity is also mentioned: for, while punishment was inflicted on one man’s crime, all others were inspired with terror. Thus, the death of one is a wholesome discipline for all, by way of example.