I have read quite a few of these, and I'm sorry if there have already been some others in the middle area that I skipped that said the same thing but here it goes.
Horses are not humans...they are animals. I love y horse, but at least when there was slaughter good horses were bought, adn bad ones were put out of their misery because it was worth it to sell them while they still had weight to make the money from meat.
I know it sounds heartless, but really would you rather have these horses starve/be abandoned or put out of their misery quickly. Slaughter is not a pretty thing, but neither is euthanasia. Depending on the age, weight, etc. of a horse they may scream and thrash when given an injection. At least slaughter gives an outlet for unwanted horses that can be strictly controlled by the government.
In my area, if you go to an auction to buy a horse to save it you had better have your trailer locked. At the end of the sale horses will be loaded into random people's trailers or left tied in the auction house. These horses have no one to care for them and many of these people can't afford to take care of them. Most of the rescues are full as well. Yes, you may have gone to help one horse, but almost no one has an unlimited supply of money to care for all the horses you would like to.
I woudl love to save all the horses I could, but all we can afford is one more. That is what we save and we pray for quick deaths instead of starvation or the long travel to Mexico for the rest.