If there were breeders doing it just for slaughter, there would be no need for those horses to go to auction. The killers would go to them and keep it out of the lime light.
If you've ever been to an auction that had a reputation for killer buyers, such as New Holland in PA (where I used to go quite often), the horses in the killer pens were not fat and healthy looking for the very most part. They were the castaways that no one wanted. They were skinny, lame, old and their bid price took into consideration the price per pound.
There was an old warning that we used to hear when selling a horse, "if the person calling about your horse seems more interested in their weight then anything else, it's probably a killer buyer".