Of all the Jews murdered throughout history, why does Israel fast today for Gdaliah ben Aḥikam? There must be a significant reason beyond the assassination itself. To answer this question, it's important to adopt a higher and more nuanced understanding that enables us to view events from the perspectives of both Gdaliah and his assassin, Yishmael ben Netaniah, who likely saw himself acting justly on behalf of his people.
While some might claim that because Yishmael made an alliance with another regional king (Baalis of Ammon), he had forfeited his right to attack Gdaliah's connection to the Babylonian Emperor Nevudkhadnetzar. But a deeper look reveals that Yishmael's alliance with Baalis was in order to initiate a shared Judean-Ammonite struggle against the Babylonian Empire - the motivation for which would have been liberating Judea from foreign rule. An alliance with Ammon based on intersecting anti-imperialist interests cannot be compared to Gdaliah representing Babylonian interests in Judea. As a descendent of King David fighting for Hebrew independence, Yishmael likely felt a patriotic duty to cut down the emperor's Jewish client governor. But pure motivations and a just cause still carry an obligation to see a larger picture and the potential consequences of zealous actions.
From the other side, Gdaliah was simply trying to do the best he could under the conditions that existed. He was close to Yirmiyahu (who David's descendants, their supporters and likely also the Babylonians mistook for a traitor) and was being pragmatic under the circumstances. Because he really wasn't a traitor and was actually trying to prevent Judean society from falling apart following a major national catastrophe, it was probably difficult for him to understand that others viewed him as such. This would explain why his guard was down - he and Yishmael likely saw their disagreement in a very different light.
The Rambam teaches in Hilkhot Taanit 5:2 that what we are really fasting over on Tzom Gdaliah is the complete loss of Hebrew sovereignty. Despite being a puppet governor appointed by the foreign emperor who had just destroyed Jerusalem's Temple and exiled her people, Gdaliah was - according to the Rambam - the last ember of Judean independence.
We mourn Gdaliah's death on the third of Tishrei each year (or on the fourth when the third falls out on Shabbat) because - despite him being a vassal appointed by a hated foreign ruler - he represented the last tiny thread of Hebrew sovereignty in our land.
But just as it was clearly a mistake for Yishmael and his supporters to demonize Gdaliah then, it might also be a mistake for us to vilify Yishmael now. The primary message we should take away from today's fast is the need to appreciate the spiritual value of the Jewish state we currently have, despite its limitations and the sometimes disappointing behavior of Israeli political leaders. According to the Rambam's position on Tzom Gdaliah, even a small thread of Hebrew independence has spiritual value.