I'm NOT arguing here, but just wondering. If you get say wild horse (btw, I don't see anything romantic about getting wild horse, but only problems unless you have tons of experience to deal with one) or horse with problems (for example, seller lied or you took one out of pity (YES, it's NOT the way to go, but things happen)), and horse is too much for you AND noone is interested even in taking it for free. Now what should you do? Put it down? Sell it to the meat plant?
My last horse (the one I lost last May) was sold to me as being ready to ride...I soon found out the contrary, that he was no where near being ready to ride, and was in fact fearful of everything and anything to do with human beings. Well...fortunately for me, that didn't matter a whole lot, as I had wanted a project horse anyway. I hadn't bargained for having to totally retrain him, but that's what I did...I went back and trained him like as if no one had ever touched him before, and I did it slowly; I figured, it had taken them 5 years to get him this fearful, there was no way I was going to undo it in just a few weeks...well, okay, I could have, but why bother? I had his whole life to spend enjoying him, so why rush things? So I spent the next 10 months reintroducing him to everything, and finally after owning him for 11 months, I finally presumed him to be ready, mentally, to ride...we never looked back, he was the best trail horse I have owned. So to answer your question, if I wind up with a horse with issues that I may not have been told, I try my best to slowly work through them...if I win, like I did with Pride, fabulous...if I don't, then I would look at finding another person who may have more experience than me in dealing with that particular issue or behavior in a horse and sell, or place the horse with full disclosure.
Now, I've never experienced a horse that is 'unfixable' quite yet, but I guess if I ever did, the most humane thing I could think of would be to put it down...I couldn't, personally, place a horse with issues that could get someone killed no matter if they knew about them or not; if the horse is so damaged on the inside that there's no hope for him to come around, I would rather know what happened to him than to place him with someone else, and potentially start a cycle of him being handed from person to person, possibly being abused and neglected in the process. I know some people are opposed to putting an animal to sleep that is 'healthy' physically but I think an animal that is unstable mentally is unhealthy as well, and if it is so bad to the extent that is impunes that animal's quality of life, I think it's up to us to decide what's best for him.