What do we learn from "va'Ya'amdu Bnei Amon u'Mo'av Al Yoshevei Har Se'ir Lehacharim u'Lehashmid"?
'Rashi': Hashem arranged that they fight and destroy each other, until they were all dead without a remnant.
Malbim: When Mo'av saw that Amon was weakened, Mo'av wanted to weaken Se'ir. They made Shalom with the remnant of Amon, on condition that they help to fight against Har Se'ir. They totally destroyed Se'ir. Mo'av had left a remnant from Amon to help Mo'av against Se'ir 1 .
What do we learn from "Azru Ish b'Re'ehu l'Mashchis"?
Malbim #1: Then it became known that one helped the other in order to destroy. Since Se'ir helped [Mo'av] against Amon, and afterwards Amon helped [Mo'av] against Se'ir, Mo'av destroyed them. Also, there was frenzy among them, and man killed his neighbor.
Malbim #2: At the time of frenzy, the allies (refer to 20:2:1:2) killed each other.
