Haman had been exalted above every other man in the kingdom, excepting Ahasuerus. He had riches, glory, honor. Everywhere he went he was revered. Yet, he allowed one man to prevent him from enjoying this.
Obviously, I believe that Mordecai was right not to bow. Even by modern, American standards we wouldn't want someone to go against his belief system in such a fashion. But, regardless of whether he were right or wrong, Haman could have disregarded his affront. When everyone else is bowing to him, why even notice Mordecai.
The truth is that Haman was a drug addict, and his drug of choice was pride. The higher he rose on it, the more he would fall when the high dissolved.
We used to work with teens in our youth group at church, and more than once we helped with all-night activities. The first one we did I consumed as much caffeine as I could, thinking it would help me survive the night. By the end of the evening every part of my body felt heavy. I went home to sleep and the sleep wasn't pleasant, I was exhausted for days. The last one we chaperoned I couldn't consume caffeine, because I was nursing my older daughter, who is very sensitive to it. I thought the evening would be difficult, but I drank water, and noticed that I felt fine by morning. I slept for two hours and got up to play with my girl! What a difference! The first night I was so high that the fall was torturous. So was Haman's pride, he flew so high on it that when he saw Mordecai he fell hard. If he were not sustaining himself on his pride but chose the much healthier option of humility, we may never have had this book of the Bible!