The reason given by R' Yehoshua ben Korchah for Esther inviting Haman to a party was in order that Achasverush should suspect her and Haman to be intamately involved and Haman and maybe her as well will be killed. As Rashi explains, that if Haman would be killed the decree to kill the Jews would die with him! (Taanis 29a).
The fact is, that Haman was killed! So why did the gezeirah not die with him?!
Kol Tuv
alex lebovits, toronto, canada
Rashi in Ta'anis (29a) says that the reason why the Gezeirah is cancelled when one person dies is because the others involved assume that the death is due to the fact that they made an unjust decree. Esther hoped that if Haman were to be killed, his co-conspirators would see this as a bad omen and retract the decree. However, this did not transpire and despite Haman's death, the decree was not annulled.
Dov Freedman
I am not happy with Reb Dov's answer. According to that logic, Esther was going to give up her life for a plan that might not even work. Besides, Alex's question is not only from the fact that in practice the Gezeirah was not repealed, but from the verse in Megilas Esther, where Achashveirosh says "...For what was written in the name of the king and signed with the signet of the king cannot be returned" (Esther 8:8). According to Reb Dov, it can be repealed, but it was decided not to repeal it.