All Jr's guilty of is tumbling on over a jump gone wrong. There's a lot of acts of malice horses can be guilty of, but that accident isn't one of them.
So people were warned to give him a wide berth in the aisle. Lots of stallions can prone to biting, particularly if they're bored in a stall and kids might be likely to go give him treats if they weren't warned to stay away.
There were two people at that farm who had permission to handle him. At a barn full of lesson kids, I don't see anything unusual about that. Doesn't mean he wasn't good for anything but Alpo. Jeepers, ladies!
I gotta say, Junior was one of the most beautiful horses I'd ever seen. He was a gorgeous rich red-brown - I called him bay, but I don't know now if he might not have been chestnut, technically - with not a white hair on him.
He'd have been beautiful breeding stock. I don't remember enough about him to judge whether he had an actual temperament disorder or was just bored.
Would you have kept him at your stables?
It would create a liability problem in the future for his owners if the action was deliberate.