Rav Isaac Sher the Rosh Yeshiva of Slabodka in Leket Sichos Mussar offers his explanation to this age old question. He says that according to Rav Shimon Bar Yochai the aveira of the Yehudim was that they bowed down to Nevuchadnetzar's idol. Even though it technically wasn't Avodah Zara they should have been Moser Nefesh like Chanania, Mishael, and Azarya. Their tshuva was to serve Hashem until the point of dying Al Kiddush Hashem which they did after Haman's decree was signed.
Similarly our avodah on Purim is to imagine ourselves willing to die Al Kiddush Hashem. The gemara says in Sanhedrin (43a) that when someone is sentenced to death by Bais Din, on their way to their execution they are given wine in order that they should be intoxicated and not feel pain. So too us since we are prepared to literally give up our lives we drink wine to ease our pain.
This explains why after getting drunk Raba slaughtered Rav Zeira. Since they both had in their minds that they were dying, Raba got a little carried away as the line between reality and fiction blurred to the point where he carried out his holy Kavanos.
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