Because now Voldemort has what he'd consider ironclad proof that Snape is on his side. However, the good guys will never trust him again, and will probably kill him on sight. Hmm. I wonder if there is some reason Harry needs to believe he's bad (if he isn't), or if he just hates Harry that much.
Just finished the book, and I'm completely up-in-the-air on this. I still believe that Dumbledore was right about Snape, and therefore this probably had to happen. Teh Obi-Wan effect, maybe. When did Snape get the most upset? When Harry called him a coward. Which makes me think that maybe he's being very brave, and would like to be recognized as such, but can't. He'll definitely be in the confidences (such as they are) of Voldemort and the Death Eaters now. But if Dumbledore saw this coming, and Snape was supposed to Kadavra him, and none of the Order believes Snape, how will Snape (eventually) aid in the destruction of Voldemort? Surely he won't be reporting to Potter, even if he could convince him, which he couldn't now anyway. So, either I am wrong about Snape, or else maybe Dumbledore somehow returns, even as a ghost or something, to explain it to Harry? Hell, a letter might be enough.

Also, if their formal education is over, how will they continue to train up for battle? We need a montage training scene. Especially the younger wizards. And Neville.