Whom did Haman plot to destroy throughout the kingdom of Xerxes?

Answer: All the Jews

Scorning to lay hands on Mordecai alone, Haman sought to destroy all of Mordecai's people, the Jews, throughout the kingdom.

“And when he learned the identity of Mordecai’s people, he scorned the notion of laying hands on Mordecai alone. Instead, he sought to destroy all of Mordecai’s people, the Jews, throughout the kingdom of Xerxes.”— Esther 3:6 (BSB)