I do have a feeling that Snape is doing it to push Harry - how better to push him than to have Snape throw it in his face that he can't do it well in the way that Snape can do so well. It leads me to believe that Snape still may be good. But, the down fall for thinking that is that I wonder if Snape won't ever kill Harry because Lord Voldemort wants to do it himself and any who cross him will die probably with a lot of pain.