The answer is: Mordecai would not bow down to him

Esther 3:5-6

Haman’s vendetta against the Jewish people offers a fascinating glimpse into ancient power dynamics and prejudice. Haman was the highest-ranking official in the Persian Empire, and his position came with expectations of respect and adoration—mostly represented by bowing. The fact that Mordecai, a devout Jew, refused to bow down ignited Haman's rage, illustrating how his egotism intertwined with a monumental disdain for those he deemed inferior. It's illuminating to note that the act of bowing was more than just physical submission; it had profound implications about loyalty and allegiance during those times.

What we see here is a classic, albeit tragic, narrative - a single individual’s defiance threatens to unleash chaos on an entire group. Haman, consumed by arrogance, decided that one man's refusal should cost millions their lives. His edict wasn’t just an attack on Mordecai but also an assault on Jewish identity itself, revealing the deeply entrenched antisemitism of the time.

As a curious side note, every Purim, a Jewish festival, celebrates the victory over Haman's plot through the reading of the Book of Esther, known for its themes of bravery and resilience. This feast signifies not just survival, but the triumph of good over evil, a topic prevalent in countless narratives across various cultures. Haman's story has echoed through the ages, serving as a reminder of the perils of pride and the importance of standing against oppression in all forms.