Even though many halachos that apply to lashon hara will be repeated in Hilchos Rechilus but it’s worth it to repeat and not have you figure it out by yourself to ensure no mistakes. Essentially rechilus is tattletaling. It is a severe sin which is the main sin of “Don’t be a tale bearer amongst your people” (Vayikra 19:16). It causes much murder amongst the Jews as we see by the case of Does HaAdomi and that is why the pasuk right after this one is “You should not stand over the blood of your friend.” Because of Doeg HaAdomi a whole city of Kobanim, Nov, was wiped out. Doeg HaAdomi told King Shaul that Achimelech gave David bread and Goliath’s sword. This normally would not have been a big deal and if Shaul would have asked Achimelech if he gave David the sword when David was found with it he would have admitted to it since Achimelech thought he was doing King Shaul a favor by giving his son-in-law, who was respected in his household bread to eat and the sword of the enemy he defeated. However, Doeg knew that Shaul had insane jealousy for David and when he told King Shaul the news he knew Shaul would get upset and he murdered a whole city who was helping David. That is the power of rechilus.
Besides this sin you are able to transgress other sins as discussed in the beginning of the sefer just like by lashon hara. What exactly constitutes rechilus? It’s peddling words from one person to another. For example, as the Sma”g (lav 9) states, If one tells you something in private about someone and then you go over to that guy and say, so and so was just talking about you and this is what he said about you (and said negatively) that is the classic example of rechilus, as the Shaarei Teshuva says (222). Also, if Reuvain says to Shimon, this is what Levi did to you, or I heard this is what Levi did to you or wants to do to you. All this constitutes rechilus assuming there is no positive purpose of telling what happened, which will be discussed in chapter 9.
What makes rechilus so bad is that if Shimon would have confronted Levi by himself then Levi might not have denied any allegations, or if Levi did nothing wrong and the truth is with him, or the intent is not what was originally thought then nothing bad could have come out. But now that Reuvain tells over the story to Shimon of what Levi said or did then he might put a negative spin on the issue and an argument will ensue or wrong implications will be concluded that will lead to fights and potential murder That is why rechilus is so bad.