Smorzie, I get your concern, it's just that the alternatives are truly often worse for horses. I am more concerned with the other end these days - people breeding horses because they want a "cute" foal, or their dog because it would be "nice" to have puppies, etc, leading to an oversupply of horses and dogs that too often end in neglect or euthanasia. A horse lives for 25-30+ years, how many people breeding casually think that far? Another bee in my bonnet is horse racing industries having 2-and 3-year old age classics and racing before horses are mature enough to stand heavy work without permanent damage - the average race horse is too injured to run competitively before age 10, which is considered "old" in such circles. Hence I joke that the "Pedigree Pal" dog food is thus called because of all the expensive pedigree racehorses it's made from.
I agree that horses need to be able to be slaughtered locally rather than transported long distances to countries that may or may not have humane slaughter practices. I also agree that we can't make double standards for different kinds of animals, but need to be concerned with the welfare of all of them while alive, including the need for humane killing. Death is an unfortunate part of life, and if you watch animal documentaries, you'll find that carnivores in nature are not generally quicker or more humane killing their prey than a trained person with a firearm.
We had to shoot a horse recently - my favourite horse ever. The state of her health necessitated it. After 31 years it was not an easy decision, but the method of killing her was quick and almost certainly painless. We are planting a tree on her, but not everyone can do that, and there is a good argument for letting the food chain continue.