3 Say to it, ‘This is what the Sovereign Lord Jehovah says: “Here I am against you, O mountainous region of Seʹir, and I will stretch out my hand against you and make you a desolate wasteland.
4 I will turn your cities into ruins, and you will become a desolate wasteland; and you will have to know that I am Jehovah.
5 For you showed unrelenting hostility, and you gave the Israelites over to the sword at the time of their disaster, at the time of their final punishment.
– Eze 35:3-4
Seir is a mountain region which were conquered and possessed by the Edomites, the secondary name of Esau (which means “red” refering to the red stew he begged from his brother Jakob).
Esau was swearing to kill his brother Jakob (Gen 27:41). The many centuries following the Edomites were not able to control their jelousy and hatred towards their “brother” Israel.
Because he pursued his own brother with the sword, And because he refused to show mercy; In his anger he keeps tearing them apart relentlessly, And he remains furious with them continually
– Am 1:11
Edomite hatred was clearly revealed as they urged on the devastators of Jerusalem (Ps 137:7), rejoiced at Judah’s tragedy, and in their enmity and desire for revenge even turned over Judean escapees for slaughter by the Babylonians. They joined other neighboring peoples in plundering the land, and they planned to take over the abandoned country of Judah and of Israel, speaking boastfully against Jehovah. For this, Jehovah directed his prophets Jeremiah, Ezekiel, and Obadiah to assure Edom that its rejoicing would be short-lived and the treatment meted out to Judah would also become Edom’s portion
Another mistake of the Edomites was to “speak arrogantly against [Jehovah]” (Eze 35:13) and ignorantly forgeting that Jehovah himself was bringing diving judgment on his people and not being unable to defend Judah.
They were also terrible things done by Israel to Edom. For example Joab (durings Davids reign) slaughter for several months all male Edomites.
However all of this was a old grudge and conflict which eventually only caused the Edomites to be completely devasted.