What will be a stumbling block?
Rashi: The counsel of Shevna and Pekach, to cease Malchus Beis David.
What is "l'Mikdash"?
Rashi: It is [preparation,] like "Hiskadashu l'Machar" (Yehoshua 3:5).
Radak: It is a castle and strong place in which one is protected, like "Ki Mikdash Melech Hu" (Amos 7:13), "u'Va El Mikdasho" (below, 16:12).
Malbim: Just like you will be Makdish Him, He will be for you a stronghold and protection from all affliction. For the conspirators, He will be a stumbling block.
What is "Even Negef"?
Rashi: It is a rock on which feet bang. It is an expression of Michshol (stumbling block), like "uv'Terem Yisnagefu Ragleichem" (Yirmeyah 13:16), "Pen Tigof ba'Even Raglecha" (Tehilim 91:12).
Malbim: Via their conspiracy, they caused Melech Ashur to come against them and exile them.
Why does it add "Tzur Michshol"?
Rashi: [Also] this is a stumbling block; Tzur is a rock.
Malbim: Its danger is greater than Even Negef. However, it itself only causes damage, unlike Even Negef, which itself damages. So in the Nimshal, the counsel of conspiracy to rebel is like Even Negef. It is a small damage, for the rebellion will not help. However, the damage that will come from it will be great like "Tzur Michshol" - Melech Ashur will come and exile them.
What are the two houses of Yisrael?
Rashi: Pekach ben Remalyahu and his supporters, and Shevna and his supporters. They came to be stumbling blocks for the residents of Yerushalayim.
Radak: One is those who trust in and desires Malchus Beis David. For them, Hashem is a castle. The other is residents of Yerushalayim who despise Malchus Beis David and choose Malchus Ben Remalyahu.
Malbim: (a) Efrayim, i.e. the 10 tribes with Pekach, and Beis Yehudah outside of Yerushalayim that Pekach conquered in the first war. Also they were against Beis David. (b) The conspirators in Yerushalayim who thought to cast off the yoke and cling to Pekach.
Why does it say "l'Fach ul'Mokesh"?
Malbim: For residents of Yerushalayim - even there, there were conspirators against their king - He will be a Pach (trap), which damages only one who enters it and is caught in it. Those who left Yerushalayim to Pekach, Melech Ashur's sword caught him. Evil will not catch those who stay in the city. The Mokesh's (snare's) damage is greater than the Pach's. The Mokesh kills and damages the living. However, in one way the Pach is worse - it traps via the hunter's strength, whereas the Mokesh damages and traps by itself, without the hunter's strength. The hunter is a mere cause. The conspiracy to leave Yerushalayim to go to Pekach was a small damage - Pekach took them for slaves. The damage due to the conspiracy - they were exiled with the 10 tribes - was a great snare. 1
When Sancheriv exiled the 10 tribes, did he not take also Pekach's slaves?! Perhaps they were freed and returned home when Pekach died; the custom was to free slaves when the king died (refer to Shemos 2:23:3:3*). Alternatively, we do not attribute Galus, which happened 17 years after Pekach's death, to the conspiracy in the days of Pekach.
