Why does it say "El Hashem ba'Mitzpah"?
Radak #1: The Shechinah dwells where all or the majority of Yisrael is - "Elokim Nitzav ba'Adas Kel" (Tehilim 82:1).
Radak #2: Mizbach Hashem was in Mitzpah and a Beis Tefilah. It is where they met when they needed to meet, like I wrote (refer to Yehoshua 11:3:3:2), due to the great salvation there when the kings gathered to fight against Yehoshua.
Were Anshei Giv'ah worthy of death?
Ramban (Bereishis 19:8): According to Torah law, no. They only had Bi'ah with a Pilegesh, who is like a Zonah. They did not intend to kill her, and she died only afterwards. She became weak from the Bi'os, and became cold and died. Since they did like Anshei Sedom, the Shevatim wanted to make a fence, so such things will not occur again. Beis Din may punish unlike Torah law for the sake of a fence (Sanhedrin 46a).
Malbim: According to Torah law, no. They wanted to sodomize the husband, but they did not do so. There is no Misah for a Pilegesh, according to the opinion that she has no Kidushin (refer to 8:31:1:1,2 and the notes there). And even according to the opinion that she has Kidushin, there were no witnesses or warning. They did not intend to kill her. Yisrael were zealous to punish them, for it was done in public; it is as if they denied [Hashem]. It was almost like Ir ha'Nidachas; they wanted to kill them for a fence. Only a king may do so, or Klal Yisrael with the Sanhedrin at their head, who are in place of a king to kill Mezid sinners, especially after a great Chilul Hashem like this. 'Edah' always refers to Yisrael gathering with the Sanhedrin at their head.
