Chafetz Chaim, Part Two, The Prohibition Against Rechilut, Principle 1
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וְדַע, דְּאִסוּר רְכִילוּת הוּא (ז) אֲפִלּוּ אִם אֵין מִתְכַּוֵּן בִּרְכִילוּתוֹ לְהַכְנִיס שִׂנְאָה בְּלִבּוֹ עַל אוֹתוֹ פְּלוֹנִי, וַאֲפִלּוּ אִם גַּם לְפִי דַּעְתּוֹ כַּדִּין עָשָׂה פְּלוֹנִי, שֶׁדִּבֵּר עָלָיו, אוֹ עָשָׂה לוֹ הַמַּעֲשֶׂה הַזֶּה. כְּגוֹן, שֶׁשִּׁמְעוֹן מוֹכִיחַ לִרְאוּבֵן, עַל מַה שֶּׁדִּבֵּר עָלָיו, אוֹ עָשָׂה לוֹ עִנְין פְּלוֹנִי וּפְלוֹנִי, וּרְאוּבֵן מִתְנַצֵּל עַצְמוֹ שֶׁהַדִּין עִמּוֹ, וְהָרְאָיָה, שֶׁגַּם יְהוּדָה אָמַר עָלָיו כֵּן, אֲפִלּוּ הָכֵי {אף על פי כן}, אִם הוּא מְשַׁעֵר, שֶׁעל יְדֵי זֶה תִּכָּנֵס שִׂנְאָה בְּלֵב שִׁמְעוֹן עַל יְהוּדָה, רָכִיל מִקְרֵי.
And know that the issur of rechiluth obtains even when the speaker does not intend in his rechiluth to inject hatred into his [the hearer's] heart against that man [who occasioned the rechiluth], and even if, in his [the speaker's] opinion, the one who said or did something against him was right — as when Shimon reproved Reuven for what he said or did to him, and Reuven justifies himself, saying that the din was with him, for Yehudah said the same thing about him [Shimon] — still, if he [Reuven] believes that because of this, hatred will be aroused in the heart of Shimon against Yehudah, he is called a talebearer.