Probably similar to South Carolina reasoning, where you gotta handcuff the dead guy you just shot 5 times in the back...
The guy who got beat-down might have deserved a couple of those, because he apparently hurt the horse by riding him over rough terrain, and probably through some nasty brush.
However, it should have ended within a minute and a half.
I live in an area with just that sort of brush. The brush won’t hurt the horse. Yeah, the guy was sawing on those reins, probably no damage to horse. And yes I had and rode horses for years in just such terrain.