Haman wanted to Kill Mordecai, Queen Esther's Uncle, because he would NOT bow down and pay homage to him after he was promoted. As all Narcissist do, he became enraged and wanted to do him in, so to speak. But he had heard that he was a Jew and all the stories of how their God fought (for THEM)...would Hurt you!
Esther 3: 3-6 in the Amp. Reads...
Haman’s Plot against the Jews
3 After these things King Ahasuerus (Xerxes) promoted Haman, the son of Hammedatha the [a]Agagite, and advanced him and [b]established his authority over all the officials who were with him. 2 All the king’s servants who were at the king’s gate [in royal service] bowed down and honored and paid homage to Haman; for this is what the king had commanded in regard to him. But Mordecai [a Jew of the tribe of Benjamin] neither bowed down nor paid homage [to him]. 3 Then the king’s servants who were at the king’s gate said to Mordecai, “Why are you disregarding the king’s command?” 4 Now it happened when they had spoken to him day after day and he would not listen to them, that they told Haman to see whether Mordecai’s reason [for his behavior] would stand [as valid]; for he had told them that he was a Jew. 5 When Haman saw that Mordecai neither bowed down nor paid homage to him, he was furious. 6 But he disdained laying hands on Mordecai alone, for they had told him who the people of Mordecai were (his nationality); so Haman determined to destroy all the Jews, the people of Mordecai, who lived throughout the kingdom of Ahasuerus.

Hamon couldn't kill Mordecai because of whose he was (one of Jehovah's chosen ones) the fear of just the rumours scared him so he couldn't do what he badly wanted to.

Is the enemy scared of you? Have you stood for God so He can stand for You?