http://query.nytimes.com/gst/fullpage.html?res=9C0CEFD81030F935A25754C0A9639C8B63

That's a link to the New York Times review of Harry Potter and the Half-Blood Prince. I hadn't read it before - even after I finished the book the first time. It's very interesting - and very true.

I still stick to my assertion that Snape isn't evil. Based on this book, I think they give us all of the elements to put together the fact that Dumbledore told Snape that, when it comes down to it, he will have to kill him in order to keep up the facade of his devotion to Voldmort. As Dumbledore said the whole book, "My life is useless." It was Harry Potter that now mattered, as he will be the one to face Voldemort in the end. Dumbledore sacrificed himself - and possibly Snape (I still don't know how that will play out) in order to give Harry the drive and ability to succeed. I think Snape might live: through mercy Harry will continue to gain his power. Voldemort may kill Snape: I don't know. I just can't seen Severus surviving to the end of the story.

But Dumbledore spends this whole time giving Harry the tools he needs to survive - showing him all the memories, telling him everything, as though he expects to die, leaving Harry in charge of the quest. He told Harry about the remaining Horcruxes so that Harry can find them himself, and allowed himself to get injured beyond repair. Then he stunned Harry so that he could not interfere when Snape came and killed him. I think Dumbledore wanted Snape - a man already responsible for death - to kill him, rather than Draco Malfoy, thus introducing an innocent to the evils of murder.

Draco couldn't do it anyway: he even said that when he was crying in the bathroom. I still have some degree of sympathy for the little dickweed, though. I don't know why.

But Dumbledore almost told Harry why he trusts Snape - in "The Seer Overheard", page 349 in the American version.