What is the significance of Haman being hanged on a tree?
The Sages taught (Ester Raba 9:2) that when Haman became infuriated with Mordechai for not bowing before him as everyone else did, his wicked wife Zeresh advised him to plot a death for Mordechai that none of his ancestors had ever escaped from. She argued, “If you cast him into a fiery furnace, Chananiya and his companions have already been saved from one. If you seize him to die in prison, Joseph has already emerged from one. If you exile him to perish in the wilderness, his forefathers have already been delivered from one. Rather hang him on a tree since we do not find that any of his people have ever escaped such a death.”
“And the matter was good before Haman and he made the tree-scaffold” (5:14).
What type of tree was it? When Haman decided to prepare the scaffold, G‑d asked all the trees which one would volunteer for the wicked Haman to be hung upon it. The fig tree said, “I offer myself since the Jewish People offer my first-fruits for the mitzva of bikurim.” The etrog said, “I offer myself since the Jews cherish me for a mitzva.” The willow said, “I offer myself since the Jews take me for the mitzva of the four species.” The vine, pomegranate and nut trees each said, “I offer myself since Israel is compared to me.” Similarly, the myrtle, olive, apple, date, palm and cedar all said, “We offer ourselves since we are compared to Israel in righteousness.”
Then the thorn came before G‑d and said, “Master of the Universe, let me who has no merit be the one upon whom hangs Haman who has no merit. Furthermore, I am a thorn and he is a thorn. It is fitting that a thorn be hung on a thorn.” G‑d complied, and the tree which Haman’s servants found with the intention of hanging Mordechai was a thorn tree.
When his servants brought the tree before Haman, he delighted himself in posturing himself toward it to demonstrate to them how Mordechai would hang from it. At that moment, a voice from Heaven called out and said, “As he gestures in jest, so this tree is in fact prepared for him. Let Haman hang from the tree he prepares for Mordechai!”