The Ibn Ezra quotes the opinion in the Talmud (Megillah 15b) of Rebbe Eliezer that the verse’s use of the phrase “in his heart” is proof that Megillas Estherwas written with Ruach HaKodesh, Divine Inspiration, (see Introduction). After all, if Haman said something to himself, how did the authors of this book know what he was thinking privately?
The Sfas Emes, however, asks why this would prove that the entire book is written with Divine Inspiration. It should only prove that its authors, Esther and Mordechai, had Divine Inspiration. He answers that people with Ruach HaKodesh would not have written that they have it in such an obvious way. They would not show it off.
A story is told of the Rubshitzer Rav that somebody came to visit him for a bracha, but arrived at a time at which the Rav did not accept visitors. The visitor told the attending gabbai, who happened to be the Rav’s son, that he was a very important person. After the gabbai persisted in explaining that this was not the time for the Rav to receive visitors, this guest asked for a glass of water. When the gabbai gave him the water, the man said he could not drink from the cup because he could tell that it had not yet be toveled (submerged) in a mikvah since he could feel the Name of H-Shem on it. Indeed, upon investigation, the Rav’s son learned that the cup he had given the man came from a box of vessels awaiting mikvah immersion. Impressed with the man’s obvious holiness, he rushed him in to see his father, the Rubshitzer Rav. After the visit, the Rav asked the gabbai why he allowed the guest at a time when the Rav did not see visitors. His son told him then what had transpired earlier, and concluded, “I pray that one day, I am on the level to see if a cup has been toveled merely by holding it.” The Rav responded, “When you become that great, I pray you are the type of person who will not tell that to the person who gave you the glass.”
The Midrash (Esther Rabba 10:3) writes that evil people are controlled by their hearts, and brings several proofs from TaNaCh of this. Righteous people, on the contrary, are in control of their hearts.
Rebbetzin Heller explains that righteous people are are rational, and evil people are emotional. The Maharal adds that an evil person’s whole being is focused on the temporary, terrestrial world, whereas the righteous are focused on the permanent, the holy – not controlled by the momentary whims and passions of their hearts.
Parenthetically, it is amazing to note that Haman who had arrived with the plan to kill Mordechai, could switch gears without missing a beat, from murder to honoring himself. Perhaps this is because both come from the same negative inclination/characteristic.