Honestly, 'humane slaughter' is kind of an oxymoron.....Killing isn't nice...it's neccesary...You can't chemically put down an animal intended for human cunsumption, a real bullet is more than a little impractical than a bolt gun, and while there 'may' be 'some' sadists in the industry, the industry it's self isn't sadistic and those individuals who do show that they are otherwise 'inhumane' to the livestock being culled should be prosecuted.
Not only that, but the bolt, even if fired incorrectly (which can happen, granted) the time it takes to kill the animal is NOTHING as inhumane as leaving the slaughterhouses closed and leaving the horses to starve to death or be shipped to Mexico where TRUE inhumane slaughter practices are happening.
Or, what about the people who are so desperate to sell a horse and can't, so they just let it out at some park or on the side of the road (It HAS happened!!!) Know what happens then? Road kill...true sadists get a hold of them and do truly evil things...packs of dogs or other wild animals introduce themselves as part of the horse's food chain...and the horse will always be at the bottom of it.