I think others have given you some good advise so I'm not going to repeat it.
Something that goes through my mind as I read this is- what is going on inside this horse's head to cause him to behave this way. Whatever it is it, it might be something that tourments him, or causes him to be a "tortured soul" if one could put a human inflection on it.
You said he had a "switch" of sorts, and it seems there are other red flags about this horse.
As owners of animals, sometimes we have to make tough choices for our animals because they can't make those choices for themselves.
Sometimes euthanasia really is the kindest choice we can make.
Terrible wounds and injuries can certainly make euthanasia the obvious choice at times of trauma, but as owners we sometimes need to make the same choice for inner injuries or trauma because it may be the right thing to do for the animal.
Hugs. It is a difficult place to be in.