I was in a large animal practice for eight years and occasionally had to euthanize horses. I tried to make the process as peaceful as possible, often putting the horse under surgical anesthesia before euthanasia. The usual method of euthanasia was a high concentration of barbiturates.
With veterinary induced euthanasia the horse is treated gently, and a needle is slipped carefully into its vein. If the horse is too wild or reacting too violently from a painful injury or medical condition, he is given the anesthetic drug in the muscle. Either way, there is no rush to finish killing the animal. The drug is allowed to put the horse in an anesthetized state so that an IV needle can be inserted in its vein. The euthanasia material is then administered until the horse dies.
In a slaughter plant, processing speed is more important than keeping the horse calm or treating the horse gently. The animals are stressed from the time they are unloaded. Horses are easily frightened, and in a loud slaughter plant where they are rapidly herded into runs leading to the stunning area they are very scared. Sometimes the holding pens are dirty and not protected from the weather. Sometimes the horses are injured, sick or old. They should be handled gently, with special care and sensitivity.
Slaughter plants do not consider the condition of the horse. The horse is just meat on the hoof to be killed, processed, and sold as fast as the process will allow. I would never send one of my horses to a slaughterhouse, and I would never suggest that a client do so.