My ingenious theory- Snape is a dirty filthy traitor who should die a most painful death (this was my theory even before he killed Dumbly). As for Dumbly, he is going to stay dead. Before his death, Harry just sat around waiting for Voldy to attack. He is now setting off to kill Voldy on his own. There is only one book left. If Harry is to kill Voldy, then there isn't much room for Dumbly to return, is there? Unless he pulls an Obi-Wan thing...