The word "Purim" is old Persian for "lots". The whole manner in which a lot (or lottery) works transcends rationality. Take, for example, the Temple service of Yom Kippur: a lot was cast to determine which of two goats would be pushed off the cliff to certain death, and which would have the distinction of being offered on the altar. Before the lot was cast, it could go either way: neither goat was logically more "fit" for one or the other of the two fates. Only the lot decided; the entire affair was altogether beyond intellect. Ultimately, the Jew is connected to G‑d in the same superrational way…
Ultimately, the Jew is connected to G‑d in the same superrational way. Despite whatever inconsistencies there may be in someone's conscious or rational relationship with Him, the essential, intrinsic. supperrational connection always exists; one can always fall back on it. By tapping into it, all shortcomings can be overlooked and forgiven.
However, if there is a level at which evil is overlooked and doesn't matter, it would seem that the forces of evil could also draw sustenance from that level, once the pipelines of life-force are opened. Haman knew that there is a superrational level of relationship to G‑d, from which good and evil can draw equally. This is why he cast lots when devising his scheme to wipe out the Jews. He wanted to "imitate" the Jews' relationship with G‑d; to "rise" to that level where intellect has no say and cannot deny him the power he wants.
Esther realized what Haman didn't. She knew that there are two facets of the superconscious. True, G‑d is above good and evil; His essence exists outside the context of creation, and "As for your many sins - can they harm Him?...and if you acted righteously - will it benefit Him?" (Job 35:6-7).
But this is only when we take reality at face value. What we see is that G‑d gives life to all Creation, and no moral strings seem to be attached. Good and evil seem to be equally capable of receiving, and there appears to be no advantage in being aligned with one or the other. The Jew knows to look behind the facade…
But the Jew knows to look behind the facade, behind the "mask" and "costume" the world is wearing. The Jew knows that G‑d created the world for the purpose of ultimately revealing Himself in it, and whatever creative force he invests in keeping the external forces of the world operative if there only to serve the higher purpose. G‑d needs a running world to serve as a backdrop for the drama of human history, He gives life-force to the world regardless of its moral performance. But that life-force is only the external dimension of His energy and interest in the world.
G‑d's inner intention is that His presence be revealed in the world and this is possible only through holiness, Torah, mitzvot, and Jews. On this level, the highest of all, evil has no "equal rights." A dog may be able to eat from a king's table, but only what is thrown "over the shoulder." the scraps. To actually sit down and dine at a royal feast, one must be refined, one must be worthy. Esther knew this, and took advantage of Haman's ascending to the vantage point where rationality has no sway in order to reveal his true colors.
In order to prevent Haman from succeeding, Esther did two things. First, she and Mordechai inspired the Jewish people to do teshuva: Then Esther told them to tell Mordechai: "Go gather all the Jews in Shushan and fast for me, and neither eat nor drink for three days, nights and day: Also I and my maidens will fast..." (Esther 4:15-16). It was necessary for the Jewish people to delve into themselves and reveal a deeper connection to G‑d than that normally operative; they had to rise to the level of His innermost Will. This is the inner meaning of the continuation of the verse: "....and so I will go into the king, though it is against the law." The soul ascends to its most intimate level of communion with the King, higher than rationality- "against the 'law'".
Once that was done, Esther invited Haman to a feast with the king (Esther 5:4). By according him the distinction of dining with the royal couple, she led him to believe that he was in fact on their level. Now that she and her people were holding by the inner dimension of G‑d's will, she was able to elevate Haman to that level as well. At that dramatic point, the king realized Haman's true intentions to destroy the Queen and rape her in front of him. When Haman's ultimate nerve was exposed, he was thrown to the gallows.
Interestingly, it is in this verse that G‑d's name (which is not mentioned explicitly in the entire book of Esther) appears as the initials of the words "Let the king and Haman come today...."
By revealing within himself his absolute and essential bond with G‑d, the Jew wipes out any possibility of evil getting out of hand. This is the message of Purim: by taking all things to their root, the Jew eradicates evil automatically.
[Adapted by Moshe Yakov Wisnefsky from Ma'amarei Admor HaZaken (discourses of the Alter Rebbe), p.381]