There is literally nothing you could have done. You did your best, you can't control people, you can't expect your words to actually work....when one's mind is a mess, and when one has made a decision....there is no way of helping other than physical restraint. It happened, you can't change it. You can mourn, you can look back and see if there is anything you can learn in case, God forbid, something like this ever happens again. But, you CANNOT blame yourself in anyway. You are not magic, you don't control reality, you can only do what you can do, and do your best.