Charles Ellicott 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)

Your hand shall be first upon him to put him to death. —A law tending to prevent false accusation. Where the witness is obliged to carry out the sentence he demands himself, or to aid in carrying it out, secret accusation is impossible; and it is far less easy to pervert the law in order to prosecute a private quarrel.