I think that they told you he was euth so you wouldn't be so heartbroken. They were trying to be nice. No one wants a young, healthy horse to die, but accidents happen.
I'm sorry for the horse, but you shouldn't keep thinking abot the most horrible ways he might have died. You don't know, you can't know. Let yourself mourn for your loss, but don't try to imagine the worst things, he's dead anyway and he can't suffer no more.