“The king then removed his signet ring from his hand and gave it to Haman the son of Hamdasa the Agagi, the oppressor of the Jews.” (Esther 3:10)
The king then removed his signet ring. Giving the signet ring was giving over all the powers of the kingdom. With the ring, one could sign any new law into existence and Achashverosh gave over all that power to Haman. Achashverosh could have insisted that Haman formulate the decree and bring it to him for inspection and confirmation, after which Achashverosh could have signed it into law — without giving Haman all that power. Instead, Achashverosh, in his desire to kill Klal Yisrael, and in his desire to have the job completed quickly and efficiently, gives Haman absolute control and power. Achashverosh even let Haman keep the ring for further use, so that he would be able to create any decree which would help facilitate the destruction of the Jewish nation. If one considers how reluctant people are generally to give away even small amounts of power, even to those whom they trust (to how many people does one give away his credit card or the keys to his safe?), one can appreciate the unlimited enthusiasm Achashverosh had for Haman's plot. Haman was probably thrilled beyond belief. He had reckoned on having to pay huge sums of money just for Achashverosh’s agreement, and not only didn’t he have to pay, but he also obtained Achashverosh’s total power.
Even Klal Yisrael were greatly shocked by the giving over of the ring, and they began to repent. The Gemara makes an unprecedented comment regarding this action, saying that the giving over of the ring had a greater effect on Klal Yisrael than did the forty-eight prophets and seven prophetesses who exhorted Klal Yisrael to repent, yet were unable to bring Klal Yisrael to complete repentance. Klal Yisrael were utterly shocked and scared when they heard that Achashverosh, to whom they had tried to be loyal and had attended his banquet, had given over all his power to Haman their archenemy, so they turned to HaShem for salvation and repented.
This episode is another indication of HaShem’s Master plan in the great overturning of the Purim story. Whatever Haman and his cohorts hoped to be a source of disaster and ruin for Klal Yisrael ultimately became part of their great salvation. Even giving over the ring was eventually good for Klal Yisrael. Since Haman had written and signed the letters, it was easy for Achashverosh to relinquish his role in the plot and pretend that it had all been Haman’s idea. This meant that once Haman was killed and Achashverosh remembered Mordechai’s loyalty to him and knew that Esther was Jewish, it was easy for him to overturn the decree and place all the blame for the decree on Haman. He then managed to convince everyone that he really loved Klal Yisrael, and as we will see, this helped greatly in the battle. Had he signed the letters as Haman had asked for, it would have been much harder for him to change sides and pretend that he had nothing to do with it. Furthermore, it would have been clear to everyone that Achashverosh did not really like the Jews and would not be unhappy to see them die. When the battle would have occurred, many of the king’s soldiers would have joined Klal Yisrael’s enemies, knowing that their king would be happy that they are being killed.
HaShem also arranged that these letters were signed by the highest authority possible the king's ring. This way it was impossible — or at least not Achashverosh’s preferred plan of action — to have them cancelled. Had Haman signed these letters, it would have been easier for Achashverosh just to annul the decree and then no battle would have occurred. However, since the letters could not be cancelled, a second letter was dispatched, giving Klal Yisrael carte blanche to fight back. During the ensuing battle, thousands of those who hated Klal Yisrael were killed.