What is the significance of taking off his ring?
Torah Temimah citing Megilah 14a: This was greater than the 55 Nevi'im that prophesized to Yisrael, for the Nevi'im did not succeed to make Yisrael do Teshuvah, whereas the removal of the ring made Yisrael repent.
Rashi: This is granting a great request from the king. One who has the ring in his hand has authority over every matter of the king.
Malbim: Achashverosh is not to blame. He was tricked. Had he himself signed the letters, he would have seen that they say to kill, and he would not have agreed. However, he gave his ring, and Haman wrote to kill them, unlike he told him.
Why does it say "Tzorer ha'Yehudim"?
Vilna Gaon: It is because the nation was not mentioned 1 .
Malbim: He is to blame for the decree to kill the Yehudim, and not the king (refer to 3:10:1:3).
We already know from verse 6 that Haman wanted to eradicate all the Yehudim! Perhaps this hints that the king should have realized Haman's intent. (PF)