- Maamar Mordechai points out that, when the Jews were in Egypt, the ten plagues occurred for one month each. That being the case, the second to last plague, that of darkness, happened one month before Passover, which would mean it fell in Adar. Haman assumed the darkness was a plague that hurt the Jews since so many of them died then (see Rashi to Shemos 10:22 and 13:18). After all, four fifths of the Jews died in Egypt because they did not believe in their upcoming rescue. H-Shem killed these unfortunates during the plague of darkness to avoid the Egyptians seeing this, and assuming the Jews’ G-d is no longer with them.1 The Jews in Persia, by attending Achashverosh’s party, indicated that they, too, lost faith in their redemption, and this is why the lots falling on Adar so pleased Haman.
- Adar is also the month when Moshe died. According to the Talmud (Megillah 13b), Haman knew this because it is so written in the end of Devarim (34:8) and can be calculated from the book of Yehoshua. According to our tradition, the seventh of Adar, his date of death, is also his date of birth. Rabbi Mendel Weinbach writes that Haman did not know this because, as opposed to his date of birth, his date of death is only found in the Oral Torah.
- The Abudraham calculates that Adar 13 would mark the end of the seven-day mourning period (shiva) for Moshe. According to the Maharsha, that seven-day period of mourning continues in some mystical way the merits of the mourned. After that point, the dead only receive merit of others step up to take over their spiritual roles. Interestingly, Rabbi Dovid Feinstein notes that, Moshe having been born on 7 Adar, his bris (circumcision) would have been on 14 Adar, Purim!2
1It bespeaks a certain callousness that the Egyptians seemed not to notice the sudden disappearance of several million people.
2However, since Moshe was born complete and circumcised (Talmud, Sotah 12a), his bris would only require a symbolic pin-prick of blood called “hatafas dam bris” (Shulchan Aruch Yoreh Deah 262:1 and 264:1), and this procedure would not be help on a Shabbos (Shulchan Aruch Yoreh Deah 260:2 and 263:1). Therefore, Moshe’s symbolic bris was held on the following day, Shushan Purim.