Just my thoughts....
I am a vegetarian. I'm not against consuming animals when they are humanely treated, humanely raised (not feed lots, force fed...), and they are humanely slaughtered (not a factory slaughter house which causes so much pain and suffering).
All life is sacred. The way to stop slaughtering? STOP BREEDING YOUR CRAPPY MARES AND STALLIONS. This is a great blog that demonstrates how I feel: http://fuglyhorseoftheday.blogspot.com/
If you want your horse to have a wonderful life keep it for it's life as it's the only guarantee.
There would be a lot less slaughtering if we were more selective in breeding our horses. Don't breed a horse just to do it. Select a great mare and stallion (at least with great conformation, but also with great performance records, among a million other reasons. A registered horse has a greater chance of survival.
It's all the backyard breeding, and the Thoroughbreds that are pushed too hard, too young, that flood our slaughter houses. Geld your colts!!
As long as breeding doesn't remain selective, it will continue.
As for abused and neglected horses...that is ABSOLUTELY POSITIVELY NO REASON to submit it to that kind of ending. If the horse has been abused, that is yet another reason that humanity has failed that horse. Neglected too. Don't own a horse if you can't take care of it. These horses deserve just as much as a fit well fed show horse or your pet, they are no less of a creature because of their past.
Slaughtering of all types needs to stop or move to a more humane level.
The number of horses in the world is based on the humans. Stop breeding unselectively. If you can't sell your foal, then you shouldn't be breeding.
Sorry....very passionate about the subject.