Proverbs 10:18 (ESV)

18 The one who conceals hatred has lying lips, and whoever utters slander is a fool.

Those who falsely praise people when speaking directly to them, and who spread slander behind their backs, thus reveal hidden hatred for others. They are fools to attempt to hide that hatred, and bigger fools to carry it in their hearts. The grace of the Lord removes a desire to hurt people from our hearts, replacing it with the peaceful fruit of righteous love.