You shall kill him: the Hebrew uses a compound verbal phrase, “killing you shall kill him.” This is a way of being emphatic: “You must certainly kill him.” How this is to be done follows.

Your hand shall be first: this is to be a public execution, with the person who was enticed taking the lead. He will be the first to throw the stones, and then the others will do the same. It may be necessary to make it clear in verse 9 itself that public execution by stoning is meant, as Good News Translation does with “Kill him! Be the first to stone him, and then let everyone else stone him too.”

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on Deuteronomy. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .