“In the fortress Shushan lived a Jew by the name of Mordecai, son of Jair son of Shimei son of Kish, a Benjaminite. [Kish] had been exiled from Jerusalem in the group that was carried into exile along with King Jeconiah of Judah, which had been driven into exile by King Nebuchadnezzar of Babylon.— He was foster father to Hadassah—that is, Esther—his uncle’s daughter, for she had neither father nor mother. The maiden was shapely and beautiful; and when her father and mother died, Mordecai adopted her as his own daughter.” Esther 2/5-7 Tanakh JPS 1985
“But he disdained to lay hands on Mordecai alone; having been told who Mordecai’s people were, Haman plotted to do away with all the Jews, Mordecai’s people, throughout the kingdom of Ahasuerus.” Esther 3/6 Tanakh JPS 1985
The book of Esther is the only biblical book that does not mention G-d by name. It records no miracles and no direct intervention of G-d at all. In Esther’s story, the Lord redeems His people through the faith and courage of one strategically placed woman and her cousin. All the while, things are happening behind the scenes to bring about the final result.
This is the first time in the Tanakh the term “Jew” is used and it occurs during the second exile of the people Israel from the lands promised to Abraham. Mordecai the Jew is of the tribe of Benjamin and his people are the Jews. The name seems to be a derivative form of the word “Judah”.
The kingdom of Ahasuerus is the Persian empire and includes all of Babylonia which is within Assyria where all of the tribes of Israel had been deported to at one time or another, the last being Judah and Benjamin. (Jerusalem is in the lands allotted to the tribe of Benjamin).
Haman plotting to do away with all Jews throughout the Kingdom of Persia was a plot against all of the people of Israel in those lands. All of the tribes including the priestly tribe of Levi which is thirteen tribes though only twelve were allotted lands of Abraham.
There is no reason to believe that Haman was plotting to do away with only the people of the tribe of Benjamin. The children of Israel are the Jewish people. The Jews are not just people of the tribe of Benjamin.