I have never had that reaction to the end of a story before. It was equal parts "What? No! Don't stop there!" and "BWAHAHAHA (aka evil cackle)"! I was both thrilled and dismayed with your ambiguous ending. Which I think means it was effective.
It was a wonderful story, with Harry getting weaker as Snape waited for his rescue. Very interesting method of haunting his dreams...and his showers. Those would be the places I'd haunt, too!
I loved the snarky asides once Snape awoke. Very well done.