If you r horse is too ill too old too lame you have 3 choices, let it die naturally (painfull), have it euthanized (painfull) have it slaughtered ( painfull).
Why do you say humane euthanasia is painful?
I am curious how many people on this forum have actually received any training in regards to the slaughter process, or even euthanasia for that matter. There are indicators we look for to ensure that these animals pass in a painless, humane manner. Spasms are normal and do not indicate pain. Instead, look for a "righting response" when the animal is hung for bleeding. This means, if the animal is truly unconscious, it might thrash but the neck will remain limp because the animal is not trying to right itself.
It is normal for an animal to be rendered unconscious, not killed, prior to slaughter. This facilitates proper bleeding. I am not against slaughter, but it needs to be humane. Most slaughter facilities are not designed for horses. They are designed for cattle, which think differently than horses. Cattle should never see people in a good slaughter facility, and should be relatively quiet as this indicates they are not overly stressed.
Proper restraint ensures that whatever method of rendering insensible to pain is used, the animal will go down with one hit. A squeeze chute for cattle should close from both sides not one side. Do horses respond to squeeze chute? I don't know, I haven't seen any research on that subject and I have never put one of my horses in a squeeze chute. I have been to large slaughter plants when the pneumatic captive bolt gun was defective so they had to hit the animal a second time, with a backup, and it was very quick, I don't think it caused undue stress.
I encourage everyone to do thorough research on slaughter and euthanasia. It doesn't have to be a terrible thing. We as humans have the power to "play God" by ending suffering, and euthanasia is truly the final act of love, not something to be feared. I wouldn't personally eat a horse that hasn't been raised for meat, but who am I to judge people that do. As long as the animal is humanely killed, I see nothing wrong with not letting the flesh go to waste.