What will be like sorcery, and to whom?
Rashi: What Nebuchadnetzar did will be like sorcery, to Yisrael.
Why is it like a vain Kesem?
Rashi: They will not believe that he will succeed.
Radak: Yisrael considered it a vain Kesem; they relied on the false Nevi'im, who promised them that the city will not be given to Melech Bavel.
Radak citing his father: The Kasdim thought that the sorcery is false, for they intended [to fight] Bnei Amon, and Hashem drew them to Yerushalayim, for He did not delay [punishing] for Tzidkiyah's Chilul Hashem (he transgressed his Shevu'ah).
Malbim: Anshei Yerushalayim considered it so.
It is written ki'Ksom, and we pronounce it ki'Ksam!
Radak: It is Makor (like an infinitive).
What do we learn from "Shevu'ei Shevu'os"?
Rashi: He divined 49 times 1 , and the outcome was all the same.
Radak: It is as if the Kasdim swore to them many Shevu'os. Shevu'ei is an adjective with the same grammatical form as 'ha'Shechunei ba'Ahalim" (Shoftim 8:11).
Radak citing his father: Refer to 21:28:2:3.
Malbim: Even though the Kesem was about seven matters (refer to 21:27:1:2), and for each he cast it seven times (they were used to doing such things seven times), and all 49 times the result was the same, they considered it vain.
Radak: Targum Yonasan explains like "Shiv'ah Shavu'os" (Devarim 16:9).
What is "Mazkir Avon Lehitafes"?
Rashi: When Nebuchadnetzar did his sorcery, he mentioned the sins of Yisrael, so they would be seized in his hand. Yisrael's neighbors Amon and Mo'av told him the sins; they had heard the Nevi'im's rebuke.
Radak: This mentioned their sin, to be seized in it. Their king Chizkiyah did not guard his Shevu'ah to Melech Bavel ? "v'Gam b'Melech Nebuchadnetzar Marad Asher Hishbi'o bEi'lokim" (Divrei ha'Yamim II, 36:13). How do they expect that the Kasdim will guard for them their Shevu'ah, even if they swore to them, for 1 [Yisrael] do not keep their Shevu'os ? "Ki Mipnei Eleh Avlah ha'Aretz", "Ratzo'ach v'Na'of v'Hishava la'Sheker" (Yirmeyah 23:10, 7:9).
Radak citing his father: These are other sins (other than Shevu'as Sheker)
Malbim #1: Doing the Kesem seven times aroused mention of Yisrael's sin of Bitul Shabbos and Shemitah, and profaned the seventh. For this they were obligated Galus 70 years ? "Az Tirtzeh ha'Aretz Es Shabsoseha" (Vayikra 26:34).
Malbim #2: "V'Hinei Hikisi Chapi El Bitz'ech? v'Al Damech" (22:13) ? striking the palm (the final decree) is due to Betza, theft and murder. 'Akeh Chapi El Kapi va'Hanichosi Chamasi" (21:22) ? the decree that the Kesem will fall on Yerushalayim was due to theft and murder. The next Perek lists seven abominations of Yerushalayim connected to theft and murder ? "Lema'an Shefach Dam" (6), Oshek of converts, affliction of orphans and widows (7), Rechilus in order to kill (9), bribery in order to kill, Ribis 2 and Oshek (12). Also the curses in Vayikra are seven for your seven sins, seven times.