I don't see what is so terrible about the body going for food. Once the horse is dead, what difference does it make what happens to it? Let me clairify and say that as it stands none of my personal horses have gone to slaughter. But if a horse is dangerous and the slaughter is humane (Canada is not so bad, a quick bullet to the head) I don't have a problem with it. Many say that the euthenasia that the vets do is slow, and not always an easy death. Also, once the chemicals are used, a horse can not be buried, or used for meat. The only means of disposal is cremation. Which is not always easily done.
I live in the US and have seen firsthand what happens when slaughter is outlawed. There are worse things than death.