What is the meaning of "mi'Yrushalayim Tetzei She'eris"?
Radak (Melachim II, 19:31 citing Targum Yonasan): The rest of the Tzadikim will come out, and the refuge of those who fulfill Torah, will be from mount Tziyon.
Malbim: The horde of people in Yerushalayim remained in their place, and were not exiled.
What is the meaning of "u'Fleitah me'Har Tziyon"?
Malbim: Through Har Tziyon, i.e. Chizkiyah sitting in the fortress of Tziyon, will come out the remnant. The remnant of the exiles will come to be protected under the shade of Chizkiyah.
Why did He say "Kin'as Hashem Ta'aseh Zos"?
Rashi: Hashem will do so amidst zealousness for His name. It is not due to Your merit. We infer that Zechus Avos ended (Shabbos 55a).
Radak (Melachim II, 19:31 citing Targum Yonasan): The word of Hashem Tzevakos will do this.
Malbim: Because Sancheriv blasphemed, Hashem was zealous for His honor and His land.
