The King's Feast

The Book of Esther begins by describing a lavish feast thrown by King Xerxes for his subjects. Like everyone else, the Jewish citizens of the realm attended. The sages assert that the genocidal decree against the Jews that Haman later enacted (with the approval of the king) was the result of ordinary Jews' attending the feast.
According to the Talmud, King Xerxes was celebrating the conclusion of 70 years since the beginning of the exile from Judea. Since the prophet Jeremiah had prophesized that the Babylonian exile would last for 70 years (counting from the destruction of the First Temple, not from the initial stage of the exile), Xerxes concluded (wrongly) that the victory over the Jews could now be deemed complete and final. For his celebratory feast, he used the sacred vessels from the Temple, and came dressed in the garments of the High Priest.
The sages are quick to point out that kosher food was served to the Jewish citizens. No laws of the Torah were transgressed at the feast. Yet, the sages maintain, the punishment for attending was a decree of extermination from which the Jews only narrowly escaped. Why?
Attendance at King Xerxes' feast was a Hillul Hashem. While the Temple and its vessels were meant to sanctify the mundane constituents of the physical world, Xerxes' feast did precisely the opposite: By using the sacred vessels for mundane purposes, the party degraded what was holy. The Jews should have responded to this sacrilege by mourning and distancing themselves. Instead, the temptation to attend a party at the palace overcame them. Their attendance was an implicit endorsement of Xerxes' worldview, a world in which God was conspicuously absent.
According to the Talmud, Hillul Hashem is the hardest sin to atone for, because, by its very nature, its effect is so widespread that it is virtually impossible to undo the damage. Once the stone has been thrown into the pond, who could possibly stop the ripples?