Hilchos Rechilus, chapter 3, section 1

It is forbidden to speak rechilus even if it is completely true, even if the person is not there, and even if you know you would be willing to say it right in front of him.

It is even worse to boldly say to his face: "You said such and such about so-and-so," or "You did such and such to so-and-so." This creates even more hatred. The person who was spoken about becomes sure it must be true, because otherwise you would not have the nerve to say it to his face.