R’ Moshe David Valle writes that Achashverosh seems surprised by the decree he permitted less than a week earlier simply because H-Shem created forgetfulness (see the Dubno Maggid’s explanation of Devarim 32:18). Being able to forget can sometimes be a blessing.
Similarly, R’ Yehonason Eibshutz says it was the king’s foolishness that caused him to forget. Accordingly, this is another root of the custom to drink ad d’lo yada on Purim (Megillah 7b).
The Beis HaLevi writes that Achashverosh did not know that the decree was meant to destroy the Jews, but thought it was supposed to only assimilate them. After all, the word l’avdam could mean “cause them to be lost.” This is why Haman emphasized the Jews’ strangeness (Esther 3:8-9). The solution for “fixing” a group of people who are “weird” is to acculturate them into society. Also, this is the reason Haman said (Esther 3:11) “la’asot bo” (“to do with”) rather than “la’asot lo” (“to do to”) the Jews. This implied that if the Jews refused, they would be punished, but the punishment was not the focus. However, in the decree he wrote (Esther 3:13), Haman emphasized the punishment. This is why Esther (Esther 7:4) first notes this punishment in her plea to the king.
The Ohel Moshe quotes the Be’er Yosef that there was a fundamental difference between Haman and Achashverosh – Haman knew that the king was only interested in having the Jews conform to his society’s norms, but he likewise knew that the Jews would sacrifice their lives to avoid conversion. For many Jews, an order to change was a death sentence for the Jews. This is the way he put on a show that left Achashverosh in a state of confusion.
R’ Avraham Yehoshua Heshel (the Apter Rav) tells a story about the time of R’ Sherira Gaon (father of R’ Hai Gaon) of Pumbedisa (906-1006 CE). There were two brothers fighting over their late father’s estate. One got a Torah scroll actually written by Ezra the Scribe, while the other brother got everything else in the estate. Parenthetically, it is a beautiful thing that the one who got all of the father’s possessions would be willing to give them up for a Torah scroll. Be that as it may, one evil man who thought it was ridiculous that the brothers were fighting over what he considered nothing more that a large parchment with ink on it, came into shul at night and scratched out the letter ayin in the word vi’avadetem (“and you shall serve”) in the verse (Shemos 23:25), and wrote an aleph in its place, turning the word into vi’avadetem (“and He will destroy you”). When this change was discovered, the owner of the scroll fell ill. He then had a dream in which his father appeared. His father told him the culprit will lose his eye because of the verse (Shemos 21:24) “ayin tachat ayin” (“an eye for an eye”) can be homiletically interpreted as an eye in place of a letter ayin. Since the scroll’s owner was concerned lest another scribe fix a scroll written by Ezra, the father calmed him by saying that Ezra in Heaven would fix it. Indeed, the next morning, the recovered owner came to shul, and together with the congregation was astounded to find the scroll in its original form. The Apter Rav brings this story as proof that Achashverosh really wanted the Jews to be subservient to him, but Haman used the word li’avdem to intentionally mislead the king into signing a decree to kill them all.