Okay, I only watched the video and skipped over much of the following thread, though I did get the gist of most of it.
That is not the first time that horse has kicked. A first time kicker will generally not aim with such clear deliberation and precise targeting. Therefore, the horse is a kicker. It will happen again, perhaps next time he'll catch her in the face or the neck or the back where there is nothing to protect her. Unfortunately, it may take a crushed face or a broken spine for her to realize that the horse is troubled.
Frankly, I don't really see that as worthy of the "you need to wear a helmet" argument. To me, that video is more proof of "you need to learn how to train/handle a horse properly".
I can't say for certain that I would definitely put the horse down for that, but that would certainly be the final option if I deemed that the horse was not going to be able to be trained out of that. I won't keep a kicker and, if he wouldn't stop, I couldn't sell him in good conscience. Therefore, I'd have him put down...or shipped off to slaughter.
With the horse market like it is, I've got no use for a horse I can't trust and I'd never trust that horse even if he never offered to kick again. I wouldn't want to keep him, but at the same time, I wouldn't dare try to sell him without full disclosure about his issues which would include a history as a kicker. That alone would knock out much of the limited market for a horse like him. So, he's essentially worthless.