Musar su Ester 7:78

In fact, Queen Esther would have liked to have evoked pure mercy, for the Divine name in its simple form [i.e., and not as an acronym] and in proper sequence is a Name of mercy. But if Esther had not mentioned this Name in an acronym of initial letters she would have been unable to defeat Haman. In her wisdom she knew that she must incorporate a small amount of judgment as well as mercy in order that this judgement would take revenge upon Haman, oppressor of the Jews. After this it is stated, “For he saw that the evil was concluded against him from the king” — Ki ra’ah kY chalsaH eilaV hara’aH mei’eis hamelech (Esther 7:7), the “king” being an allusion to the King of Universe. Note that the Divine name appears here in proper sequence indicating mercy for Israel. Nevertheless, it appears as an acronym of final letters to hint at the judgment and destruction that was to befall Haman and his sons. From here we can see the extent of the wicked Haman’s evil intentions towards Israel. In him was fulfilled the verse, “He excavated a pit and dug it out” (Tehillim 7:16). That is, he was ensnared by the very trap that he laid. Therefore the Torah has commanded us to wipe out the name of Amaleik and his seed and the seed of his seed. Thus the Sages have taught (Sifri, Devarim 25:19; Mechilta DeRashbi, Shemos 17:14) that one must blot out the name of Amaleik even from upon the trees and stones. This is the source of the custom — and the customs of Israel are Torah — to bang upon hearing Haman’s name, in order to wipe out his name and that of Amaleik. All the authorities, both earlier and later, have written that one must not abolish this custom.