The gemara (Rosh Hashana 18b) says that we have 3 fasts directly related to the Churban (Asara B'teves, Tisha B'av, Shiva Asar B'tammuz) and the 4th fast of this series is the fast for the murder of Gedalia Ben Achikam the leader of the remnants of the Jews in Eretz Yisroel after the Churban Bayis Rishon. The gemara explains that the reason we fast for Gedalia is to teach us that the death of tzaddikim is on par with the destruction of the Bais HaMikdash. Why Gedalia? There have been many tzaddikim murdered. Why not fast for them as well?
The Ben Yeho'ada answers that the murder of Gedalia was not a Gezeira Min HaShamayim. It was avoidable since it was committed by a Jew, Yishmael Ben Netanya. When a Jew is murdered by a goy that is a gezeira but murder by a fellow Jew comes from "bechira ra'a" and is not a heavenly decree (except for certain instances). Moreover says the Ben Yeho'ada this caused a great Chilul Hashem. The evil Nevuchadnetzar was kind enough to leave Jews alive in their land and it was us that were the bad guys.
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