According to the Ben Ish Chai, the verse emphasizes that Haman prepared the gallows on which he dies because if the wood of the gallows was made from the beams of the Beis HaMikdash, and the Halacha as brought down by the Rambam (Mishneh Torah, Hilchos Shegagos 9:1) would not allow Mordechai to make use of it. However, since the wood of this beam has already been used by Haman, this removed its sanctity, making it usable to kill him.
According to the Ora V’Simcha the gematria of ha’eitz (“the tree”) (5+70+90=165) is the same as Haman (5+40+50=95) + 70. Seventy are the number of days Haman was in power. According to the Chida, seventy is also the number of verses between Haman’s rise to power (Esther 3:1) until his downfall (Esther 7:10). Finally, seventy is also the gematria of yayin (“wine”) (10+10+50=70). The very wine with which Haman intended to seduce the Jews of Persia to sap them of their spiritual power is what led to his undoing. This may be yet another reason for the Talmudic custom (Megillah 7b) to drink an unusually large amount of wine on Purim.
R’ Yechezkiel Levenstein writes that many people recognize that their suffering comes from their own sins, but they do not realize that the sin, itself, creates the punishment.
The Alshich writes that Esther said she will fast “similarly,” or more accurately, “with you” because of the oft-mentioned Talmudic concept (Baba Kama 92a) that when a person needs something, and decides to also pray for another who also needs that, this person’s prayer becomes more effective.
Ben Ish Chai again points out that the gematria of kein (“so”) (20+50=70) is seventy, the amount of hours Esther would fast.
The Beis Yaakov quotes a Talmud (source unclear) that bad breath becomes strongest after 72 hours, and this is why Esther does not fast for 72 hours straight.
The Eshkol HaKofer writes that if Achashverosh had not commanded artificial respect for Haman, he would not have received it organically. Either the people did not like Haman, as we shall see below (6:3), or as the Eshkol HaKofer suggests, they saw him for what he truly was.
After all, the Yalkut Shimoni and the Targum Sheini write that Haman was originally a barber – a low position in Persian culture as it implied, besides cutting hair, more menial tasks like removing warts, bleeding, etc. A person would hardly bow to such a person in those times unless commanded otherwise by the crown.
The M’nos HaLevi notes that the word “chein” (“so”) used here to describe the king’s command, has the gematria of 70 (20+50). Again, this represents the peak of Haman’s power because that is how long he was in power.