Musar על סנהדרין 206:13

Esther realised that the Jews were more fragmented than they had ever been in their history, and she considered this a major cause of the disaster facing them. This is why she counteracted Haman by calling for the unification of her people. She was certain that the fragmentation of the Jewish people stemmed from their laxity in religious observance. We are told in the Midrash Esther Rabbah 7,18 that eighteen thousand five hundred Jews of Shushan participated in the feast king Ahasverus gave for the population of Shushan. According to this Midrash, Mordechai had warned the Jews not to attend the feast, and had told them that they had only been invited in order that their free-will participation would be used by Satan as an accusation against them before the Celestial Tribunal. The Jews did not heed Mordechai's warning, however. This demonstrated that whereas the Jews were fragmented in their relations with G–d and the Torah, they were united when seeking rapprochement with their Gentile rulers and neighbours. We know from Sanhedrin 103 that eating and drinking creates separation between those who had been close prior to their feast, whereas it brings together people who had previously been strangers. This is why the sin of the Jews who attended the feast was many times greater than it had been before when they were estranged from G–d without having become particularly close to the Gentiles. The only way to rehabilitate themselves was by means of another meal, the one Esther prepared for Ahasverus and Haman, which aimed at reversing the process of becoming close with the Gentiles. This is why the miracle of Haman being hanged occurred during the meal.