As sad as horse slaughter is, it is a humane option. Think about it this way... If people were no longer able to send their horses to slaughter, what would happen to the hundreds of thousands of horses sent each year? Short answer, being shot, left to starve, beaten to death etc.... There doesn't seem to be any good answer here.
I find it so sad that so many beautiful horses are killed each year but even more upsetting is them being loaded into cattle cars and driven hundreds or thousands of miles to Mexico then standing them out on kill farms starving or with broken bones or other injuring waiting to be killed. As sad as it is, it is much more humane to end their suffering here, and doing it quickly. The way they do it here beats other methods used elsewhere.
There is nothing wrong with having strong emotions over this subject as it shows you have a big heart. Save the horses you can. Encourage others to adopt vs. breed when possible etc... Beyond that, work to do the most humane thing one can do at the end. Educate all those that you can.