I guess since I've already subbed this topic by posting, I might as well offer my two cents, which is simple:
If a horse puts me or my family at a higher-than-normal risk, it goes. Hopefully, that will mean rehoming, but sometimes it doesn't.
I have my limit on behaviors - if it's something I don't feel confident handling, but could be safely fixed, I will pay the money for the horse to be trained by a professional trainer or re-home permanently with someone who can safely handle it and that I trust would not unload the horse on someone who would be in danger. This is not a horse that I would try to get money out of nor sell to just anyone.
I also have my limit on cost. I cannot afford a pasture pet at this point in my life. If I cannot re-home nor pay to fix the behavior, euthanasia is on the table and will only be chosen after much thought and consideration primarily for the horse and the ability for that horse to be cared for properly and not end up in a kill pen. If I cannot guarantee the horse a happy life here on earth, then it'll go be happy somewhere else where it won't be putting my family at risk.
I don't see this as being much different than my limit on vet bills either. I know that I can afford up to about $1000, maybe even $1500 in special circumstances, to take care of my baby. But should the estimated costs of treatment exceed that or the value of the horse (which is inflated, of course, my my own bias of how wonderful and amazing of a horse he is), then I'm prepared to take the less expensive option of euthanasia rather than put my family at financial risk.
I love horses enough to know when to have them PTS. It's a much kinder alternative to ending up in a kill pen, which is still kinder than becoming unloved and neglected.