Figure Analysis
Ah, Haman—he's the wicked Wormtongue whispering in the ear of Ahasuerus (kudos if you got our Lord of the Rings reference) and but one in a long line of evil counselors who eventually get their comeuppance. Jaffar in Aladdin springs immediately to mind. Will they never learn?
No one in Persia seems to have cared too much about the fact that the Jews in the Persian Empire were worshipping a different God and celebrating different customs until Haman comes along, rubbing his hands in a quite evil way and snickering (probably). But Haman wants to kill all of them just because one person—Mordecai—refuses to bow down to Haman in the street. So Haman is both petty and a genocidal madman—not to mention greedy. He's not only going to kill all the Jews, he's going to plunder them afterwards as well.
Super Villain Size
To let you know just how bad Haman is, the author describes how totally massive Haman's gallows for executing Mordecai will be: about 75 feet. Now that's just a needlessly high gallows. (Though, to be fair, it was sort of his friends and wife's idea.) But because Haman's so dramatically bad, he can't do anything at a chilled-out, normal, or moderate level. It can't just be a standard-sized execution device; it needs to be a super-villain-sized one.
Things are very up-and-down for Haman. After the first banquet with Esther and the king, he's in a good mood—then he sees Mordecai, who refuses to bow to him again, and he gets in a bad mood. His wife and friends advise him to build the super-gallows, which puts him back in a happier frame of mind. He's still in a good mood because he thinks the king's going to award him—but then the king awards Mordecai.
Haman, despite his pretensions to power and control, is really at the mercy of everything that's going on around him. His moods and his sense of himself are dictated by the position of power he's in—but that's so fragile, so much at the whim of little things (like one person refusing to bow) that it doesn't bring him any happiness or lasting contentment. Thus, he ends up doing all these horrible things instead of just accepting the fact that he's always going to be at least a little powerless in relation to the ways of the world as a whole.
Key Differences
In a way, that's a key difference between the two Persian holders of power Haman and King Ahasuerus, and Esther and Mordecai. Esther and Mordecai are both very much in control of themselves, with certain ideas about the kinds of things they will and will not do. Mordecai is sure of himself and his principles and hence won't bow down to a Persian counselor like Haman. For her part, Esther's sense of herself and her ability to win the favor of people helps facilitate her rise through the harem.
On the other hand, King Ahasuerus blindly accepts Haman's suggestion to murder the Jews—almost without thinking—and Haman's own reactions are determined by little flicks and fidgets in his outer circumstances (like Mordecai not bowing to him). In Esther 5:12-13, Haman says, "Even Queen Esther let no one but myself come with the king to the banquet that she prepared. Tomorrow also I am invited by her, together with the king. Yet all this does me no good so long as I see the Jew Mordecai sitting at the king's gate."
Self-Reliance
Even though The Book of Esther doesn't mention God or get too religious, you could say that part of the reason for all this is because Esther and Mordecai are sure of their religious identity, their footing in front of God. They're confident and self-possessed and basically make all the right moves.
Neither Ahasuerus nor Haman have this kind of self-possession. The power and the plunder they hold are more important to their own self-estimate, which is why Ahasuerus wants to show off his first queen to the people inappropriately, and why Haman is so vindictive towards people who question his power. Their sense of self is based on what they own, not on what they are.
This is important for understanding why Haman is a villain—or, at least, it's a good psychological guess, given that the Book of Esther doesn't go too deeply into its characters' psychology and leaves the readers to figure it out for themselves. (Don't worry, though. Shmoop's got your back.)