How she [Jerusalem] sits in isolation! (Lamentations 1:1).
The opening verse of the book of Scriptures that depicts the fall of Jerusalem cites a state of isolation. Badad connotes loneliness, abandonment, and the state of being shunned by others. This term also appears in the Torah in regard to the expulsion of a metzora (someone who suffers from a disease called tzaraas), who is to be isolated from the community (Leviticus 13:46).
The Talmud states that the affliction of the metzora is in retribution for the sin of lashon hara. Indulging in harmful talk brings about enmity and divisiveness. Gossip and slander can turn people against one another and sow suspicion where once there had been trust and friendship.
The Talmud states that when Jews were united, and when there was no lashon hara among them, they were triumphant, even though they were far from perfect in other respects. On the other hand, when lashon hara causes dissension, all other merits may not suffice to tip the scales.
On the ninth day of Av, Jerusalem became badad, shunned by its neighbors, shunned its former friends, and to all outward appearances, even shunned by God. Why? Like the metzora, the Israelites had been guilty of behavior that brought about divisiveness. By bringing about the state of badad within their ranks, they themselves became badad, isolated from God.
We must jettison all personal whims and desires that stand in the way of Jewish unity, for in unity lies our salvation.