Yossipon writes that Achashverosh went to the garden to confirm the information he just acquired with Mordechai.
The Talmud (Megillah 16a) writes that this verse specifically mentions Achashverosh’s garden because Achashverosh’s was equally angry in his return as he was in leaving the room. What angered him while he was away was the sight of angels who looked to him like people. One Midrash (Esther Rabba 10:9) adds that it was the angel, Michael, and another Midrash (Pirkei D’Rebbe Eliezer 50) says that they looked like specific people – namely, Haman’s sons. These angels were cutting down the trees of his garden. Astonished that they would be cutting perfectly good trees from his own garden, Achashverosh asked them,” What are you doing?!” They responded, “Haman ordered us to do this.”
The Vilna Gaon asks why the story would require this entire incident with the angels. This is an especially cogent question when one considers the Aggadic story’s element of seeming dishonesty; after all, Haman did not order anyone to cut the king’s trees. He answers that H-Shem is treating Haman like he treated the Jews, mida kineged mida, measure for measure. Just as Haman maligned the Jews (Esther 3:8), H-Shem treated him the same way, having angels lying about Haman.
The Maharal adds that they looked like Haman’s sons cutting down good trees to show Achashverosh what kind of person Haman is. After all, the Jews, who are compared to trees, are good, and undeserving of being cut down.