Hilchos Rechilus, chapter 1, section 1 2

Chapter 1

This chapter explains: the prohibition of rechilus even when it is completely true, what counts as rechilus, what to answer if someone asks what another person said about him, and all the other details of this prohibition.

1) Speaking rechilus breaks the negative commandment "Do not go tale bearing in your nation." This is a great sin and has caused the death of many Jews. That is why the Torah continues: "Do not stand by the blood of your fellow." See how the rechilus of Do'eg the Edomite caused the death of the people of Nov, the town of kohanim. Besides this clear negative commandment, you also transgress many other negative and positive commandments, as explained in the introduction.

2) What is rechilus? Carrying words from one person to another, saying: "So-and-so said this about you," "He did this to you," or "I heard he did this to you, or wants to do it."

This is rechilus even if what you tell is not negative, even in your own opinion. And it is rechilus even if that person would admit saying or doing it if asked, because he thinks he was right, or because he had a good reason for what he did.