One of the themes of the Megillah is the idea of אחדות or Jewish unity. It is interesting that the way that Haman convinced Achashveirosh to kill all the Jews is that he told the king the following: ישנו עם אחד מפוזר ומפורד בין העמים, "that there is a nation scattered and spread out among the nations."

The מעם לועז says that Achashveirosh was concerned that he will look bad if he annihilates an entire nation. Haman reassures him that the Jews are so scattered that nobody will even notice.

Esther, on the other hand, united all of the Jews. She told Mordechai, לך כנוס את היהודים, "Go and gather all of the Jews." Esther was asking for אחדות, Jewish unity. Several times in the Megillah the word ויקהלו, "and they congregated", also signifying Jewish unity.

We must never forget how strong we are as a people when we are united as איש אחד בלב אחד, one man and one heart. Purim tries to emphasize the importance of unity rather than divisiveness.