The Talmud (Megillah 7a) notes that one of the proofs that Megillas Esther was written with ruach hakodesh (see Introduction) is that no human writer could possibly know that the Jews did not take any spoils.
Rashi writes that the Jews had rights to the spoils, but decided to wave those rights, and give the spoils to the king in order to maintain friendly relations with the palace.
The Dena Pishra writes that they did not take spoils because they did not want others to think that the Jews’ motivation was financial.
In M’aarchei Lev, Rav Moshe Schwab writes that since this was the property of Amalek, it was forbidden to take, as was the case for Shaul (Shmuel 1 15:3). and this is why the Jews refrained from doing so here.
In fact, the Binyan Ariel and Nachal Eshkol write that the Jews’ self-control in this incident was a tikun for the sin of Shaul in sparing (Shmuel 1 15:9) Amalek’s property.
Interestingly, the M’lo HaOmer and Me’am Loez both note that the initial letters of the ninth, tenth, eleventh and twelfth words of this verse, uvabeeza lo shalchu es (“and from their spoils they did not send”) can be rearranged to spell Shaul.
The Sfas Emes writes that the Jews took the spoils, but destroyed them in an effort to not benefit from the property.
However, R’ Yitzchak Yeruchem Diskin writes in Ohelim that Jews have an obligation to take the property of Amalek and destroy it, but did not do so here. The reason was that the Talmud (Megillah 16a) considers Haman to have been a slave. As such, he relinquished all rights to personal belongings. This includes his children. This also answers the question of how his grandchildren could study Torah in Bnei Brak if Amalek is never allowed to join the Jewish people. Such is not the case for his grandchildren because of his status of being a slave.
Megillas Seris adds another reason they did not take the spoils – they only had one day to kill Amalek, and they did not want to run the risk of missing the opportunity to fulfill this mitzva. In the course of performing a mitzva, they totally ignored anything ancillary to killing out their enemies.
The Gerrer Rebbe notes that matanos la’evyonim, the Halachic (Shulchan Aruch Orach Chaim 695:4) injunction to donate to the poor on Purim is in honor of the impoverished Jews of the time not taking the spoils of their enemies, despite their needs.
The Midrash (Esther Rabbah 10:1) explains that Achashverosh was certain that the reason he was afraid of an assassination attempt was because he had overlooked rewarding somebody. Indeed, they found the incident earlier (Esther 2:21) when Mordechai warned the king about Bigsan and Seresh’s assassination attempt.
M’lo HaOmer points out that it is a miracle that, although Achashverosh wanted to sleep, he asked to be read a record book which would seemingly have the opposite effect. After all, he was the king, and affairs of state, regional power struggles, palace intrigue, and other similar events recorded in such a record would more likely excite the king instead of put him calmly to bed.
Yosef Lekach writes that the entire records book was simply a list of the king’s personal debts.
Ibn Ezra, however, states that the book was a list of vows, and the king was feeling guilty and was under the assumption that his insomnia was a punishment for an unfulfilled vow.
The Malbim writes that Achashverosh was frustrated about what Esther’s request might be. Therefore, he asked for the records book to investigate if Esther’s request might merely be for him to reward someone for a favor done for him.
According to the Vilna Gaon, the verse is discussing two separate books – a chronicle of the nation’s entire history, and records book of the king’s personal enactments. Similarly, the Malbim points out that the big book is a public book, available to all the citizens. The smaller book is private, and only available to the king. As we shall see (be”H), the difference is that one is more likely to be manipulated.