First upon him — God thus ordered it, for the caution of witnesses, that, if they had thro' malice or wrath accused him falsely, they might now be afraid to imbrue their hands in innocent blood; and for the security and satisfaction of the people in the execution of this punishment.
Related Commentaries of Deuteronomy 17:7
Deuteronomy 17:7
7 The hands of the witnesses shall be first upon him to put him to death, and afterward the hands of all the people. So thou shalt put the evil away from among you.