Hmm, well, personally, while I don't like the idea of slaughering a horse because he's not living up to his potential, as it sounds so ugly.BUUUUTTT,
Everything that lives must die at some point and suffering is hard to define sometimes. Being deprived of basic living rights-food, water, shelter and companionship. Yes, it's cruel. Having to suffer immense physical pain, cruel as well.
Working for a living-i.e. Racing, jumping or pulling wagon? Not cruel, if the animal is compensated for it and not abused with overwork, beatings, etc. I have to work and work hard, my body takes a beating some days, but that's life.
I don't think it's cruel for a horse to be put down, in and of itself. If the animal isn't forced to endure long standing pain during the death. Like I said, all life comes to an end at some point. I think a rubber bullet is kinder than being ripped to shreds by a predator. I think euthansia by chemical meds can inflict pain and fear as it takes longer than a bullet. Starving or freezing to death is much worse, or at least I'd prefer for someone to end it for me with a bullet than any of the other mentioned ways. I also don't see how it's cruel to be eaten. Humans don't eat horse flesh before it's dead. Once a body is dead, it's neutral. Personally, I'd RATHER them be processed and utilized one last time than to think of them rotting, and contaminating the air and water, and what do you do with the carcasses? They can't all be buried. In nature ,they would be eaten, probably the meal would start before they were completely dead.
A horse being raised for human service or entertainment is not cruelty in my eyes. Humans dominate the world, we control other species. Horse get hurt, by accident or when serving people. It's not pretty, and we should do what we can to minimize the injuries. But you can't wrap them in bubble wrap and keep them alive forever. Commercial slaughter may be scary, but horses get scared of loading in trailers, or of flapping trash bags, etc. To them, it's no less horrifying than anything else they ever percieve as danger, yet we purposely expose them to scary things to "train" them to accept them.
Horses, as much as I hate to say it, are just animals. No better than any other, we as humans favor them, so we put more value in what we think they percieve as suffering. Yet we think nothing of a cow's view on being slaughtered and eaten, or the spiders we crush on our walls. We just don't see them as valuable to us so don't think about how cruel it might be to be crushed alive..I think horses get it a little easier BECAUSE we breed them for our use. Hunters kill deer...they don't get a clean shot to the head, but we don't raise them as pets...I feel that if deer were domesticated, we would concentrate more on a "painless" death for them (and I'm in no way taking an anti-hunting stance-I am all for hunting and yes, I squash bugs) So is a horse's "slaughter" any worse than any other death? It's all relative. I'd rather slaughter them then watch them waste away with all their aches and pains.
Eating them..I couldn't eat my own horse, I'm much to sentimental. But I COULD eat a burger knowing it's horsemeat. It's just food.
I hope nobody is offended by my comments, that's just my own view.