See, the more i think about it, the more i think Snape is on Dumbledore's side. The lines about "hatred and revulsion" on Snape's face when he killed Dumbledore and later when he and Harry were fighting outside are what really sold me on the whole Evil!Snape thing, but this is UnreliableNarrator!Harry telling us these things, and people have pointed out that those emotions could be Snape's reaction to the situation -- that he hates having to kill Dumbledore (hates the whole bloody situation, with Draco trying to be a Death Eater, etc.) and then later, if he really is on Dumbledore's side, having Harry call him a coward would have stung from anyone but obviously it's worse from Harry.
He made the Unbreakable Vow, so as soon as he showed up and it was clear that Draco wasn't gonna kill Dumbledore (which i think evidences that Draco really isn't evil -- echoes back to the end of Order of the Phoenix when Harry couldn't muster the hatred to Unforgivably Curse with any strength -- which, incidentally, i thought a brilliant touch, that you can't just utter these words but you really have to mean them) he had to kill Dumbledore, and the fact that his hand twitched when he had to swear to that part of the Unbreakable Vow makes me think he really didn't want to have kill Dumbledore.
This of course begs the question of why he showed up in the first place. People have talked about him rescuing Draco, and i think that makes a lot of sense. Draco says Voldemort is gonna kill his family if he doesn't do this, and we have no reason to believe that's not true. Snape clearly favors members of his own house over students of other houses, and seems to favor Draco particularly, so i can totally see him having a particular interest in Draco's well-being. If Draco really had killed Voldemort, i wonder if Snape would have killed him (Draco) on the spot.
A huge amount of people are convinced that Snape's gonna redeem himself in Book 7 and call this a cop-out on JKR's part, but honestly i think evil!Snape has been one of her biggest failings as a writer because it seems so over-the-top (it took until Snape's Worst Memory midway Book 5 for me to really understand/believe why he was being so superlatively vile to Harry). We all know i'm a big fan of moral grey (plus frustrated by Harry -- and others -- acting upon personal feelings and not weighing this against other factors and thus making wrong decisions) and the black/white-ness of the world for most of the books was a big frustration rereading the series last week, so of course that's also a factor. I come down on the side of Snape-proving-himself-to-not-be-on-Volde
'S funny, i keep saying, "I've gone from really liking the Harry Potter books and not being interested in the fanfic, to really liking the fanfic and not being interested in the books," and yet i keep engaging with this evil?Snape question. I'm still undecided as to whether it's stuck in my head because it's an engaging question or because everyone's still discussing it and i can never resist chiming in.
Snape being the Half-Blood Prince has been stuck in my head because i recently read Andrea13's Sorting Hat series and i keep thinking, "Huh, so Tom Riddle wasn't the only non-pureblood ever to get sorted into Slytherin," so this got me thinking about why he would have gotten Sorted into Slytherin since Slytherin's dominant characteristic is ambition and when i think "Snape" i don't scream "ambition," but then i thought of how he's always after the DADA job, and i remembered how it's cursed, and i thought, "That must be why Dumbledore never gives Snape the job; he doesn't want to lose Snape" since Snape is a proven competent teacher, plus Dumbledore trusts him so he knows he's got a potential double-agent if/when Voldemort ever returns, plus he knows Slytherin's Head of House isn't a Death Eater and thus is potentially tempering Slytherin students from growing up to be Death Eaters. This then made me wonder if Dumbledore ever told Snape that he didn't give him the DADA job because it seemed cursed and he didn't want to lose him. The whole not-telling-people-things drives me crazy like whoa, so of course i hope he did tell him, but i could see Snape saying it's coincidence not causality and being certain that he would be fine and thus being bitter.
My dad (who hasn't yet read HBP) sent me a link to this article by a writer he trusts (having not yet read HBP, he also hasn't read the article). It says a lot of things i've already said, and brings up further points that make a lot of sense. I don't buy all of them -- some are major, some are incidental -- but it's a very sense-making article.
I'm not sure Ginny's gonna let Harry break up with her. Ron and Hermione insist on going with Harry, and i suspect Ginny will too. And i could totally see Harry and Ginny's interactions in Book 7 being written such that i buy the pairing, since i was unexpectedly sold on it when it happened at the end of Book 6. Except that aside from that unexpectedness, Book 6 gave me every reason to not trust JKR at all when it comes to writing romantic pairings, so i see believable Harry/Ginny far more as a fanfic possibility than a canon possibility, except that i'm likely to only read it in canon since in fanfic i'm more interested in pairing Ginny with Hermione or maybe even Luna and just ignoring Harry.