1)
What do we learn from "ki'Dvar Hashem"?
1.
Malbim: Initially, Hashem told him merely to rebuke them. Now He commanded him to prophesize about the decree. He was unsure if he should also fulfill the first command to rebuke them. The verse says that he did only what Hashem commanded now. He thought that Teshuvah will not help, and the first command was Batel.
2)
Why is it called "Ir Gedolah lEi'lokim"? They were sinners!
1.
Radak, Malbim #1: To aggrandize a matter, it is put next to Kel, like "k'Harerei Kel", "Arzei Kel" (Tehilim 36:7, 80:11), "Shalheves Kah" (Shir ha'Shirim 8:6), "Mapel Kah" (Yirmeyah 2:31).
2.
Malbim: It was important in Hashem's eyes ? therefore He did not want to destroy it!