The fact that two parashot of the Torah deal with tzara'at (usually translated as "leprosy," although the Torah refers to the physical manifestation of a spiritual disease), which is the punishment for "lashon ha-ra" (speaking badly of others), testifies to the importance that the Torah attaches to this subject. Lashon ha-ra is the only transgression that carries an immediate punishment. Let us examine some of what our sources have to tell us about the sources and severity of this sin.
In discussing the various levels of tzara'at, the Rambam (Hilkhot Tumat Tzara'at 16:10) notes that they correspond to different stages of speech defined as lashon ha-ra. He explains that if someone sits together with evil people, even if at first he talks with them about innocent matters, he will eventually come to speak lashon ha-ra about righteous people. Indeed, this is a common phenomenon – people tend to seek out bad points in others who appear better than them in order to ease their own consciences; this way, they will be able to say, "See – he's not so great either."
The Meshekh Chokhma (Parashat Beshalach, s.v. "Ve-hamayim," Shemot 14:24) explains that although the punishment for actual transgressions is more serious than the punishment for demonstrating bad character traits and negative social behavior (such as lashon ha-ra, gossip, etc.), God is more exacting concerning bad character traits when the whole generation is involved. The destruction of the Second Temple, for example, occurred in a generation that was God-fearing, studious and punctilious in religious observance – but baseless hatred was rife among them. Likewise, the great flood came specifically as a response to the generation's sin of theft ("chamas"), although they transgressed in many other areas too. (The Meshekh Chokhma lists several other examples of national punishments in response to bad character traits.)
In his introduction to Sefer Bereishit, the Netziv writes that the Second Temple was destroyed because people were intolerant, accusing anyone whose avodat Hashem (Divine service) was different from their own of being either a sectarian or an apostate. In contrast, the Patriarchs were called "upright" because they related lovingly and respectfully even toward pagans.
The Gemara (Bava Metzia 30b) teaches that Jerusalem was destroyed only because the courts would rule in exact accordance with the letter of the law, and would not demonstrate any leniency or understanding, going "beyond the letter of the law." The Ramban, too, discusses the prohibition of being a "scoundrel within the bounds of the Torah" – someone who observes the letter of the law but not its spirit. (He learns this concerning the mitzvot between man and God from the command, "You shall be holy," and concerning interpersonal mitzvot from the command, "And you shall perform the right and the good.")
Each mitzva has a formal halakhic dimension as well as an ethical dimension. Jerusalem was destroyed because its inhabitants were careful to fulfill only the dry halakhic requirements, paying no attention to the ethical aspect. A person who is strict in his observance of every detail of the laws of lashon ha-ra and gossip, but does not behave towards others as he should (for example, he is jealous of them or behaves selfishly or arrogantly), is a "scoundrel within the bounds of Torah." For this reason, it is sometimes preferable to learn a book of "mussar," such as Mesillat Yesharim, than to study the Chafetz Chaim's halakhic work on the laws of lashon ha-ra. The Chafetz Chaim's work is undoubtedly very important, demonstrating as it does that even the laws of lashon ha-ra are composed of intricate details. Nevertheless, one should not be misled into believing that the careful observance of these formal details is the whole picture – the ethical dimension should always be kept in mind.
(This sicha was delivered at seuda shelishit, Shabbat Parashat Tazria-Metzora 5753 .)