hings were not over just because Haman was dead. There was still that matter of the destruction of the Jews worldwide. The law had been published. The death of Haman essentially changed nothing in that regard. So, Ahasuerus gave his ring to Mordecai and authorized him to prevent the destruction of the Jews.
The way which he went about it was startling, but practical. He made a new law which authorized Jews to kill those who were their enemies before they had their opportunity to rise against the Jews. So, the great execution was still supposed to be on the same date; just with a very different outcome (Esther 3:13 & 8:11-12).
Things were really looking up for the Jews. Mordecai was powerful and in a high position of honor (Esther 8:15). The queen was a Jew. And, the people were happy (Esther 8:16 & 17). It was a day of good tidings.
Perhaps the most interesting result of the new law was the sudden swelling of the Jewish population. People literally became Jews. They converted to Judaism. I assume that this would have included circumcisions and some major changes in their cultural practices. But, why not? The Jews were suddenly the most influential minority in the world (Esther 8:16).