Osiris watched as Sam tried to convince Dean of his innocence. In the many centuries he had been doing this, a defendant had never been found not guilty. He couldn't let it happen now. But, Sam was good. It could happen. He could feel Dean's guilt lessening. If left up to Sam, Dean would get off. But, if left up to Dean, he would be judged and punished.
"Dean, I'm going to leave it up to you. Should I call the last witness?"
He saw it in Dean's eyes. He wasn't going to let it happen. He saw the frustration in Sam's. And he felt the glee in his own.