Even if there was no bad intent one can still transgress rechilus When he tells someone what so and so did to him or said about him. Rabbeinu Yona in Shaarei Tedhuva (74) says that a person is punished for negligence which results in lashon hara even though he had no intention of insulting the other. This would also apply to rechilus. For example, if Shimon rebukes Reuvain about what he said or did. Reuvain tries to defend himself and brings a proof because Yehuda did or said the same thing. Even if you weren’t trying to get Yehuda in trouble but if Reuvain thinks Shimon will start hating Yehuda, then that’s considered rechilus and cannot be said as a defense.
There are many proofs to this halacha. For one, the Sifri in Biha’aloscha says that Miriam did not have any malicious intent when she told Aharon that Moshe separated from his wife, still in all she was punished. The Ramban in Devarim 24:9 says straight out that even if you have no intent to do any damage it’s still rechilus. Bottom line you have to think before you talk lest you come to be negligent in speaking rechilus.
Another clear-cut proof the Chofetz Chaim brings is from a gemara in Sanhedrin 30a. When the Jewish court presents the final decision in a case, Rebbe Yochanan holds that the official written document for the decision should not spell out which judges said guilty, and which said innocent because that would be rechilus. Clearly there is no malicious intent there, of trying to malign any judges. The court statement would just say which judge said what. Still in all it is rechilus.
Another proof is from a gemara in Shabbos 33b where Rebbe Yehuda, Rebbe Yossi and Rebbe Shimon bar Yochai were sitting around shmuzing and Rebbe Yehuda ben gierim (he was a son of converts) was nearby listening. Rebbe Yehuda said that the Romans did so many wonderful things like build roads, bridges, and marketplaces. Rebbe Yossi stayed quiet. Rebbe Shimon bar Yochai said they did all that for their own benefit. Rebbe Yehuda ben Geirim told over this conversation to his students and parents. Somehow the Roman government found out about it. They rewarded Rebbe Yehuda, Rebbe Yossi went into exile and Rebbe Shimon bar Yochai was eventually executed. Before he was executed, he found Rebbe Yehuda ben Geirim one day in the market and he said what are you still doing around and he looked at him with an evil eye and Rebbe Yehuda ben geirim turned into a pile of bones. Now Rebbe Yehuda ben Geirim wasn’t trying to eat on Rebbe Shimon bar Yochai, and he was known to be a great rabbi with students but still what he said caused the execution of rebbe Shimon bar Yochai and therefore he was held liable for rechilus and deservingly punished as the Kesef Mishna points out.