I have learned that the reason for the decree against the Jews in the Purim story is that they partook of the feast of Achashverosh. Yet, I have also learned that the king had kosher food and kosher wine served for the Jews at his feast. If so, what was the reason for the decree?
You can be certain that Achashverosh did not serve kosher food and wine at his orgiastic feast out of love for the Jews and respect for Judaism!
Rather, his intention was, under the guise of a kosher-catered affair, to lure the Jews into decadence and ensnare them into transgressing in order to kindle Divine wrath against them, and prolong their exile.
In addition to hoping they’d be dragged into the obscene indulgence and licentiousness of the bacchanalian debauchery, he sought to degrade them and degrade the Holy Temple in its destruction, in order to prevent its rebuilding.
The Sages taught (Megilla 12a) that Achashverosh served the food and wine of the feast in the defiled vessels of the Holy Temple which he inherited (together with the exiled Jews) from his predecessors the Babylonians, who decades earlier had ransacked and destroyed Jerusalem. In addition to the great effrontery of reveling with these vessels in the face of the Jews, he presided over the entire orgy wearing the holy garments and breastplate of the High Priest. In this way he inflicted insult and injury on the Jews, whose forced silence bespoke apathy, and thus provoked Divine punishment.
The Talmud (ibid) also relates another reason. The students of Rabbi Shimon bar Yochai asked him why destruction was decreed upon the Jews of that generation. The rabbi asked them to venture an answer. They replied that it was because they participated in, and derived benefit from, the feast of the wicked Achashverosh. He countered that if so, only the Jews of Shushan who attended the feast should have been accountable, but not those in the rest of the provinces who were also threatened by the decree!
So the students then asked the rabbi what sin he thought caused the decree. He replied that it was on account of the Jews bowing down to the statue of Nebuchadnezzar. The students replied that in such a case, rather than wondering why the decree was made, the question should be why it was nullified such that they were saved by Divine intervention? Since they prostrated themselves to this golden image, they should be liable for the capital punishment for idolatry even if they repented!
Rabbi Shimon bar Yochai answered that they only made an outward show of bowing. Tosafot (Pesachim 53b) offers two explanations: Either the statue was made only to honor the king so their bowing only smacked of idolatry but technically wasn’t; or even if it was made to deify the king, they only bowed out of fear of death but not to worship it. Since their prostration was only external, Rabbi Shimon explained, G‑d’s display of anger was only a facade. Once they repented for this external display of infidelity, G‑d “repented” for His show of anger and spared them from death.