John Gill Commentary 2 Chronicles 20:23

"For the children of Ammon and Moab stood up against the inhabitants of mount Seir, utterly to slay and destroy them: and when they had made an end of the inhabitants of Seir, every one helped to destroy another." — 2 Chronicles 20:23 (ASV)

For the children of Ammon and Moab stood up against the
inhabitants of Mount Seir
Supposing that they had acted a treacherous part by them, and took on the side of their enemies: utterly to slay and destroy them ;
as they did: and when they had made an end of the inhabitants of Seir ;
so that there were none left of them: everyone helped to destroy one another ;
in their confusion mistook one another, either for Edomites or Jews, like the Midianites in the times of
Gideon.