Who thinks so?
Rashi: Sancheriv.
What does he not think?
Rashi: That the decree is from Me, and I send him.
Radak: That he is My agent and staff.
Malbim: That his purpose is to fix them and return them in Teshuvah, and not to eradicate them.
What is in his heart?
Rashi: He plans to destroy everything, in his pride. He does not realize that I sent him for a monetary affair, to plunder.
Radak, Malbim: He thinks that his hand's strength defeats the nations, [Radak - and he will destroy even Yerushalayim].
Malbim: He intends unlike the three limitations on his mission. (a) He does not go to destroy, only to make sinners repent - "ul'Sumo Mirmas" (6). (b) He will rule only over their money, but not over their lives - "Lishlol Shalal" (6). (c) Refer to 10:7:4:2.
Why does it say "Lo Me'at"?
Radak: After he destroyed all the kingdoms and most of Malchus Yisrael, only a little remained, i.e. Yerushalayim, that he will not conquer.
Malbim: He was sent only "b'Am Evrasi" (6), but he intends to cut off great nations, and not to cut off a little. In his eyes, Tzadik and Rasha are the same.
