Why does the 3 verse conclusion/summary in Ch 10 of Megilat Esther tell of the King raising taxes?
As Rabbi Norman Lamm has observed, the megillah is "words of peace and truth." The truth had to be put in "peaceful" terms that would not offend the government publishing this work!
There's a midrash that all this partying and skirt-chasing had drained Achashverosh's coffers, necessitating this tax. Now you couldn't say, "now the king was broke ...", so you politely wrote, "well the king levied a tax..."
(The Baal HaTurim notes this when contrasting Pharaoh's preparations during the seven years of plenty, which made Egypt an economic superpower; with Achashverosh's partying which made him broke.)
The Megillah writes that after Esther was appointed Queen, Achashveirosh suspended taxes. (Esther 2:18) At the end of the Megillah (Esther 10:1), it completes the story by reinstating taxes.
One possible explanation: One opinion holds that he discounted taxes as an incentive to Esther to reveal her nation and origin. Once she did so (in the course of the Megillah), he can now reinstate the taxes.
The Ibn Ezra (ad loc.) says that "the land and the islands of the sea" refers to lands that were technically outside of the empire of Achashveirosh. Nevertheless they capitulated to Achashveirosh and paid him tributes.
This fits the context of the following verse, which juxtaposes the power and might of the deeds of Achashveirosh with the ascendancy of Mordechai. This suggests that the success of Achashveirosh derived from the greatness of Mordechai.1
R' Dovid Feinstein (Kol Dodi on Megillas Esther, ad loc.) says that Mordechai placed a tax on the kingdom in order to remain useful to the king and to stay in the king's good graces.
1 The Malbim (ad loc.) echoes this interpretation of the Ibn Ezra but emphasizes that the successful and mighty deeds that the verses attribute to Achashveirosh were actually Mordechai's doing, and the verses merely attribute them to Achashveirosh to accord honor to the king.
The massacre at the end of the purim story presumably created a labor shortage and workers' wages went up but the number of tax payers went down. To keep the cash flow steady, taxes had to be increased. (We wouldn't want the workers to become too wealthy or they may have leisure time to write Federalist Papers and such)