Why does it say "Dirshu Es Hashem vi'Chyu"?
Malbim: Hashem warned them [to repent]. They did not do so, therefore He did not raise them from their fall. They themselves caused their fall!
What is "Pen Yitzlach k'Esh Beis Yosef"?
Rashi: Perhaps His anger will rise like fire against Beis Yosef; it is as if it says l'Veis Yosef.
Radak: Yitzlach is to cleave. The Targum of "va'Yvaka Atzei Olah" (Bereishis 22:3) is v'Tzalach. A big fire cleaves and consumes everything that it passes through. Beis Yosef is Malchus Efrayim.
Malbim: Above, it said "k'Ud Mutzal mi'Serefah v'Lo Shavtem Adai" (4:11). It was the last Makeh with which he struck them, and still they did not repent. Via this, Beis Yosef will succeed like fire to consume also the remaining spit. It will consume and not extinguish until there is no remnant. This hints to the rebellion among them in the days of Hoshe'a ben Elah, because he wanted to abolish the calf in Beis Kel. Via the rebellion, man consumed (killed) his fellow man - "va'Yhi ha'Am k'Ma'acholes Esh Ish El Achiv Lo Yachmolu; Menasheh Es Efrayim v'Efrayim Es Menasheh" (Yeshayah 9:18). A burning wood consumes the wood next to it. So they will consume each other.
