1)
Why does it say "Mipnei Asher Kitartem"?
1.
Malbim: Why did Hashem not punish immediately? This left room to err, and think that the punishment came by chance! It is because Haktarah is such a severe sin, that Hashem hides the punishment from man, and hardens his heart to make him perish.
2)
What is the meaning of "Al Ken Karas Eschem ha'Ra'ah ha'Zos ka'Yom ha'Zeh"?
1.
Malbim: For all these reasons you think that the evil comes to you by chance, and not via Hashgachah. If not [for these reasons], Hashem would punish you in a way that you would understand that the evil is not chance, rather, Hashgachah.
