Vilna Gaon: In other deaths, the body is not seen afterwards. If they hang him, they will see him constantly. They said to use a tall beam, for seeing greater than hearing in four ways. When Haman is in Beis ha'Melech, he cam see the hanging only if it is on a giant beam. Hashem caused this in order that Achashverosh could see for himself [the tree that Haman prepared for Mordechai] - hearing is not like seeing (Mechilta Yisro 19:9) - "Gam Hine ha'Etz" (7:9).

Malbim: If they would hang Mordechai for his sin itself of not bowing to him, this would diminish Haman's honor, that he quarreled with a Jew who belittled Haman's honor, Sometimes the king commands to kill one of the rebels in order to instill fear on the people, to show that one who does so will be hanged. For this they always choose one of the lowest in the nation, and hang him on a high beam, so everyone will see and fear. So they would hang Mordechai, and not for his sin against Haman.

Vilna Gaon: Go early in the morning, unlike your custom to go late. The second to the king goes last ¹ . Tomorrow, go first. Perhaps a lover of Mordechai will go to the king first and advocate for him.

Malbim: It was the practice when hanging a rebel to instill fear on the people, to do so in the morning, so immediately when people leave their houses, they will see, and not sin b'Mezid again. One who is hung for his sin, he is hung after his judgment, which lasted until afternoon, and on a low tree.

Vilna Gaon: He himself prepared the beam - love upsets the normal etiquette (Sanhedrin 105b; normally, someone as esteemed as Haman would tell a servant to do it).