It's illegal to release non-native animals into the wild. Livestock is not a native species.
It's a great idea in theory, and yes some people are doing it, but it's dangerous to the horse and people. Imagine hitting a horse while your driving down the highway in an area that should not have wild horses. Releasing non-native animals into the wild seems to be something that is enforced, not feeding livestock doesn't hold much ground unless you have a rescue group on your side.
I wish I still had the pictures I took at our local sale barn. It was disgusting. People were bringing in horses that should have been put down months ago, but there they were, laying in the kill pens waiting for a long time coming death. I got in trouble for taking pictures, but I would love to do it again some day to show why we have slaughter houses. For example, there was a horse that was litterally drug by the leg into the kill pen because at least one leg was broke. He had raw flesh from laying in feces for far too long. There was another horse whose face looked like it was peeled off and hanging from his chin, it was dry other than the puss dripping off. Many other horses laying around with what I would call feces burns because they couldn't stand up, or horses with flesh hanging from their shoulders/chests/etc. Old bandages on their legs that had grown into their flesh. It's disgusting. These horses do not go through the regular kill pen auction, they stay in the back and are bought directly from those pens. You have to go through gates, crawl over fences, and sometimes through other loose pen horses just to get to the really bad ones. They know better than to put those horses where people can see. We have stayed after dark -when they load up these horses- just to see how bad it is.
I'll try to get pictures of horses like this again and show you why I think slaughter houses are helpful. Unless we change the laws and actually send people to jail for not caring for their horses, then there HAS to be horse slaughter. It's only fair to the horses.