Esther 3:4 Now it came to pass, when they spake daily unto him, and he hearkened not unto them, that they told Haman, to see whether Mordecai's matters would stand: for he had told them that he was a Jew.

The root of bitterness springs up in people and get them defiled, make sure you always take every matter to God. Haman became defiled because some people told him something he was not even noticing. He never saw Mordecai as being arrogant or not honouring until some people pointed it to him. 

Something had happened in the  past, a seed sown because of Israel invasion into their territory, Saul did not destroy the entire city as commanded by God. So Haman was a descendant of that land, the moment some people pointed Mordecai arrogance to him, he saw an opportunity to take revenge. 

The root had always been there, he became defiled, and it led to his death. The enemy is the flesh, unresolved issues, some things that came in from childhood etc. Don't allow the enemy take advantage of you, deal with that little sin, it has capacity to grow and cause bigger problem. 

Hebrews 12:15 Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled.