John Gill Commentary Deuteronomy 27:24

"Cursed be he that smiteth his neighbor in secret. And all the people shall say, Amen." — Deuteronomy 27:24 (ASV)

Cursed [be] he that smites his neighbour secretly
And kills him, and the murder is not known; the curse of God follows such, and overtakes them sooner or later. Some interpret this of smiting with the tongue, of secret backbiting and slander; so the Targum of Jonathan, "that smites with the third tongue;" or false accusation: and all the people shall say Amen ;
as disapproving of and condemning such a practice, even slander, and especially murder.