Ester 1:5 - Mordechay is a Party Pooper

Ester 1:5: "At the end of these days, the king made a seven-day feast for all the people, great and small alike, that were found in Shushan the capital, in the courtyard of the king's inner garden."

Targum: "At the end of the days of this feast, the king made a seven-day feast for all the people of the House of Israel, great and small alike, that were found guilty in Shushan the capital, for they were counted among the uncircumcized masses, in the courtyard of the king's inner garden. There fragrant and fruit-bearing trees stood, coated halfway with fine gold, inlaid with precious stones, forming a canopy above them. The Righteous Mordechay and his colleagues, however, were not there."

The plot thickens. The Targum here introduces the cause of the oncoming threat to our existence. For those unfamiliar with the Purim story, we will here spoil a bit. In Chapter 3 of the megilah, the evil Haman persuades the king to allow him to enact a decree of extermination against the Jewish people. The Targum now explains why HaShem (G-d) allowed such events to transpire. Somehow, by the great masses of Jews of Shushan attending Achashverosh's feast, as the Targum points out, together with the gentile masses, they were "found guilty" and deserving of punishment, indeed, it seems, with their very lives.

Why was attending this feast a sin? Why so severe, in fact, that the Jews would thus deserve to be wiped out? Any suggestions?