Malbim citing Mahari: Damesek fought with Gil'ad with hot cruelty; they were listed first. After were Azah and Tzor, which helped Edom. Afterwards is Edom himself, who did evil to Yisrael. Afterwards is a punishment for the nations that caused all this. Bnei Amon caused the evil that Damesek did. They began to fight with Gil'ad and take its territory. Via this, Damesek dared to come after them and destroy. The reason for the evil of Bnei Edom, after which were drawn Peleshes and Tzor, is that [Mo'av] burned the bones of Melech Edom to plaster (refer to 2:1:2:3).

Rashi: Once, Melech Edom fell in his hands of Melech Mo'av, and they burned his bones and plastered them on the walls of the house. (Radak ? Targum Yonasan says so.) Hashem avenged their disgrace of the king.

Radak: "Va'Yikach Es Bno ha'Bechor Asher Yimloch Tachtav va'Ya'alehu Olah Al ha'Chomah va'Yhi Ketzef Gadol Al Yisrael" (Melachim II, 3:27). From then and onwards Melech Edom was angry at Yisrael, for they were with them in that war. "La'Sid' teaches that they burned him totally ? even the bones became ashes, like plaster. This is like "v'Hayu Amim Misrefos Sid" (Yeshayah 33:12).

Malbim: Edom had a Bris with Yisrael. In the days of Yehoram and Yehoshafat, Edom went with them to fight against Mo'av, and then Melech Mo'av seized the Bechor of Melech Edom destined to rule after him (therefore he is called Melech Edom). They raised him on the wall and burned him. "Va'Yhi Ketzef Gadol Al Yisrael" (Melachim II, 3:27) ? Edom was angry that Yisrael did not save them from Melech Mo'av. After that, Edom began to do evil to Yisrael, and guarded eternal hatred. Mo'av was the reason for this, therefore it was punished. It says about the fall of Mo'av "v'Hishbati l'Mo'av? Ma'aleh Vamah u'Maktir lEi'lohav" (Yirmeyah 48:35) ? Midah k'Neged Midah.