Why can Purim be one of the happiest holidays in the Jewish calendar when the Jews in King Ahasuerus' kingdom took revenge by killing more that 75,000 of their enemies (Book of Esther's 9:6, 9:16)? Of course it's a reason for joy that all the Jews were saved by Esther's and Mordechai's plot, but was not the price the people of the kingdom paid a bit stiff?
First of all, I think it should be said that the Jews didn't take revenge. They defended themselves. By Persian law, "an order written in the name of the king and signed with the king's ring can not be rescinded" (Esther 8:8). Therefore, Haman's decree to kill the Jews was never actually rescinded. Instead, Achashverosh issued a second decree in which "the king gave permission to the Jews in every city to gather and defend themselves...against any who wage war against them" (Esther 8:11).
And Purim doesn't celebrate the death of our enemies. In fact, we celebrate Purim the day after the battle ended. The Jewish way is not to revel in the death of our enemies - we celebrate the fact that God saved us from Haman's genocidal plan.
Also, the death toll may not seem so large when you consider that it accounted for Achashverosh's entire kingdom, which stretched throughout the known world.