"Even in the state you’re in you can’t let it go," he observed.  "Very well.  Sarah Stevens was in fourth year when you were in first.  She recognized me.  I had nearly managed to get out of Diagon Alley and down into Knockturn when she called to me by name.  There was nothing for it.  I had to kill her.  I could not have her telling anyone that I was back in Britain."  He looked down at her.  "This would be a lot more pleasant for you if you simply obeyed my instructions."