1)
From where will He leave?
1.
Rashi: He will leave the throne of mercy, and go to the throne of judgment.
2.
Radak: This is a metaphor for the decree going out from in front of Him. It descends and tramples on "Bamasei Aretz." Also David said "va'Yet Shamayim va'Yarad" (Shmuel II, 22:10).
3.
Malbim: This is a Mashal ? He Himself will go to punish via Hashgachah, and not via a Shali'ach, like "ka'Chatzos ha'Laylah Ani Yotzei b'Soch Mitzrayim" (Shemos 11:4).
2)
What are "Bamasei Aretz"?
1.
Rashi: The haughty people.
2.
Malbim: The tall mountains. This is an anthropomorphism (as if Hashem will stand on them).