The verse seems to intentionally obscure the identity of the letter’s author. Although the scribes were called in, it does not say who actually wrote it. The Yosef Lekach notes that the word “nichtav” (“he wrote”) is a verb grammatically in the singular form because H-Shem was the Author of the decree to destroy the Jews. In fact, as has been mentioned previously, there is a discussion in the Talmud (Megillah 12a) regarding the reason the Jews deserve such a harsh decree at this time. One opinion is that, decades earlier, the Jews bowed down to a statue of Nevuchadnetzer (Daniel 3:6). The other opinion is that the Jews enjoyed the feast celebrating their exile in the beginning of Megillas Esther.
Rav Shimon Schwab writes that the Jews then performed both of these action to maintain good relations with the nation around them – not to actually worship idols or celebrate their exile, G-d forbid. For whatever reason, H-Shem decided it was deserving, and this is the fate He sealed. He is the King referred to in this verse, Whose ring sealed the Jews’ fate. Of course, every sealed fate can still be re-opened with the power of Teshuva (Rambam, Mishneh Torah, Hilchos Teshuva 1:3).