What is the meaning of "Ki Somar Edom Rushashnu"?
Rashi: If those who destroyed the Mikdash will say, from our beginning we were poor, but now that we became rich from spoils of Yerushalayim, and we will rebuild our ruins - "Hashem says, [they will build and I will destroy]."
Radak: If Edom will say, even though we are poor and our land is desolate, we will rebuild our ruins, like Yisrael did...
Malbim: Due to say that this is not due to Hashem's hatred, rather, due to the Ma'arachah and Esav's Mazal, which does not succeed, or it is chance. We see only that Hashem does not love Esav to oversee him with special Hashgachah, and cancel for him evil of the Ma'arachah - perhaps Hashem would not actively do evil to him! Therefore, when our Mazal will rise, or we will strengthen via nature, we will rebuild our ruins! Hashem refutes this -"they will build and I will destroy." Even if nature dictates that they rebuild, I will destroy via Hashgachah amidst hatred of them.
Why does it say "v'Nashuv"?
Radak: The Vov in v'Nashuv is superfluous, like "va'Yisa Avraham Es Einav" (Bereishis 22:4) and similar verses.
Radak citing Targum Yonasan: We became poor - now we became rich, and we will rebuild the ruins.
Why will they be called "Gevul Rish'ah"?
Radak: The entire world will call their land Gevul Rish'ah, because it became permanently desolate due to the residents' evil. Their evil was great in front of Hashem - they betrayed their brothers Bnei Yakov, and Hashem commanded Yisrael "Lo Sesa'ev Edomi Ki Achicha Hu" (Devarim 23:8). They did evil to Yisrael as much as they could, and rejoiced over their Churban and Galus, like it says in Ovadyah. Therefore, Hashem hates them forever.
Malbim: There are two reasons for their destruction. (a) Opposite the Gevul of Hashem, that He put a Kadosh residence and Tzedek and commanded Brachah there, Esav put a Gevul of evil and Azazel's portion. It will always be desolate. (b) They are the nation that Hashem hates forever, via their evil deeds.
