the zechus Achashveirosh had that gave him a queen like Esther

The gemara (Meg 12b) explains why it was that Vashti was ordered to appear nude at the party of Achashveirosh . Chazal say that there was a midah k'neged midah at work here:

שבמדה שאדם מודד בה מודדין לו מלמד שהיתה ושתי הרשעה מביאה בנות ישראל ומפשיטן ערומות ועושה בהן מלאכה בשבת

The gemara then brings a source in the pesukim for this derash:

היינו דכתיב אחר הדברים האלה כשוך חמת המלך אחשורוש זכר את ושתי ואת אשר עשתה ואת אשר נגזר עליה כשם שעשתה כך נגזר עליה

There is an amazing Shem m'Shmuel which notes that when the pasuk tells זכר את ושתי ואת אשר עשתה ואת אשר נגזר עליה, we have to keep in mind the context. The megillah is not just relating to us, for our sake, the what and why of what happened. The megillah is telling us **Achashveirosh's understanding** of the what and the why. The pasuk is a window into the goings on in Achashveirosh's mind. He is the one thinking about Vashti. He is the one who was wondering why such a terrible thing should happen to his queen. And he is the one who understood that the answer is שבמדה שאדם מודד בה מודדין לו , that she was paid back exactly in kind for her crime. This was not some random event that occurred because the party got out of hand, but rather it was a precisely calibrated response to Vashti's actions.

Did you even wonder why such a simpleminded, wicked (perhaps) man like Achashveirosh was zocheh to be married to a great tzadekes like Esther haMalkah? OK, so Esther had to be in that position to effect a rescue of Klal Yisrael, but ha'yad Hashem tiktzar to put some other plan into effect?  Revach v'hatzalah yaa'mod la'Yehudim mi'makom acheir had Esther not been queen.  Says the Shem m'Shmuel: it was because of this recognition by Achashveirosh that שבמדה שאדם מודד בה מודדין לו. Esther is all about revealing the yad Hashem that is hidden in the events of history.   Megillah = giluy, to be megaleh and reveal that which is hester/ Esther. Achashveirosh on his own level had something of that midah in recognizing זכר את ושתי ואת אשר עשתה ואת אשר נגזר עליה, that שבמדה שאדם מודד בה מודדין לו.   If something happens, there is a reason behind it, there is yad Hashem that is behind it.

The holiday of Purim is about seeing the yad Hashem in the hester, in the day to day.  This year we are witness to nisim v'niflaos.  You have to be blind to not see it.  Some people are blind, and some people prefer to put on blinders.