Oh my gosh, I am of a completely different mind than all of you. I really think Snape is not evil at all, but Dumbledore's most faithful ally. I will borrow from gonnabefamous (if you want, check out her site at
thirdcorridor.proboards60.com/) because she has laid out all the points so well:
Ok... this is why I believe, and have since 4 hours after reading the book, that Snape is not evil and that Dumbledore told him to kill him.
Bear with me, this is long...
I think the reason Dumbledore didn't tell anyone else in the order is that he knew that they would stop this all from happening- I think he wanted it to. Here are my reason's behind this theory.
This would be so typical of the two of them, and if you think about it, makes alot of sense.
1) Snape didn't kill Harry in the end. He could have, but didn't. All the death eaters who have proved to be the most dedicated to Voldemort, the ones who are so hopelessly devoted to him that they would do as Snape has just done, have attempted to kill Harry. Bellatrix Lestrange tried to, Crouch tried to... it doesn't fit.
2) If Snape hadn't killed Dumbledore, then either Draco would have been forced to kill him, or wouldn't have been able to and would have been killed by Voldemort, and also, the other Death Eaters would have killed him. Dumbledore knew he was on his way out, I think he has been preparing Harry for it all year. This way, Snape will still be around, and know what is going on. He didn't think Draco could do it, and with all the aurors around, Draco couldn't have pulled it off- the unbreakable vow would have killed him.
3)The other adults are too protective to help Harry, Dumbledore would need to be able to rely on someone who wouldn't shelter him, as that has only resulted in tragedy before. Likelihood would dicate that (with Dumbledore, the only person who truly trusts Snape) gone, his duties as spy would no longer be too helpful. Snape may end up saving Harry in the end- and I agree with whoever said it before- I don't think that J.K. Rowling would really turn him into the character that is evil after all the children have been like, "he is"- the lesson isn't there, and he's too complex a character to just be like, oh, there's no big secret, he's just evil.
4) I'm starting to get the idea that this has become more of a "do whatever it takes" kind of situation.
Snape might have planned it with Dumbledore on the account of the fact that Dumbledore was a goner no matter what, and he is the one whom his loyalty lies with. Snape wouldn't really want to take orders from anyone else, and it's possible that Dumbledore figured that this way, there was no possible way that Snape could change his mind about which side he was on and corrupt the order, as Dumbledore is probably the only one who would be able to tell. Snape could now help Harry, but if he decides to work against the light, the order won't be affected- I'm sure Dumbledore would have thought of that.
5) Notice the way Snape speaks to Harry during his escape. It's the same way Dumbledore tries to dissuade Draco from killing, saying that he's not the killing type. Snape discourages Harry from using the unforgivables- he's like, "not from you, Potter, your haven't got the nerve or the ability." It's like Snape's version of what Dumbledore said to Draco.
6)Dumbledore made a major point in book 5 that there are worse things than death. I think he was talking about this, about Draco's position. It would be worse for an innocent boy to become corrupt because he is forced to do this than it would be for Snape, who has already made that mistake, to do it instead. I think J.K. purposelly made Dumbledore say, "Severus, please..." so it would be ambiguous. But you're right, and I felt the same way- Snape was different. It was like he was still trying to teach Harry a lesson. And I picked up on that torture thing too- If he was that bad that he would kill Dumbledore out of hatred, why would he have a problem letting them torture Harry? He stopped them as soon as they began. Could it be that he hopes Harry will kill Voldemort in the end? If Harry dies, Dumbledore's death will have been in vain. It wouldn't surprise me if Dumbledore had made him promise to protect him...
However, here's what worries me. Another major point made throughout the books is that you can't use an unforgivable without meaning it, without wanting to cause pain. Snape killed Dumbledore with Avada Kadavra- could that mean... or does the hatred have to be directed at the object of the attack? Well- Moody says in the fourth book that Harry could say it and he would get no more than a nosebleed. Perhaps with all Snape's experience, he could have knocked Dumbledore out without really killing him, and thats why he sent him flying over the rampart wall- to finish off the death. This way, the other Death Eaters would really think he had wanted to do him in.
7) I honestly think Snape's whole "dont call me coward" tangent could be related to his not being evil. Think, if he is still good, he just killed the only person who trusts him, who ever gave him a chance. He has lost every friend and colleague he's gained at Hogwarts over the years. He's lost the place that has been his home- the side he is really on will no longer offer him protection, and if the Death Eaters find out where his loyalties lie, he really has nowhere to go. Even if Voldemort is defeated, he has no way out. He just did something that made the difficulty of his life increase tenfold, and I think his attitude reflects obvious pain- he seems almost broken as he deflects all of Harry's spells, and it's easy to understand his rage at being attacked with his own hateful spells and being called a coward when he just did the hardest thing he'll ever do for noble reasons.
8) Ok, stop to consider the conversation Dumbledore and Snape had which Hagrid heard. Snape said that Dumbledore took too much for granted and maybe he didn't want to do it anymore. This is right after Harry is injured.
Dumbledore has taken for granted, throughout the whole book, that Harry will be able to defeat Voldemort without him. Perhaps this is what Snape is alluding to; perhaps he doesn't agree that everything will be ok with Dumbledore gone. Dumbledore reacts the same way he does when Harry doesn't want to do something if it means hurting Dumbledore; he says no, and that Snape said he would do it and that's that.
... it's not difficult to believe that Snape was willing to take the fall, and after Harry told Dumbledore what he'd heard, this is what Dumbledore deduced. He didn't say till later that he'd asked Snape to watch draco, and in that convo, he'd mentioned Snape making investigations. Maybe he was doing this so that Draco might ask Snape about it, and Snape could help him. He said Snape most likely said the thing about the unbreakable vow to Draco because of his cover when he was talking to Harry, which means he must not have known what was going on yet. He may have then gone to Snape and confronted him, figuring out that since Snape had not told him, that he was going to take the fall, since DUmbledore trusts him so much and would guess this. Snape might have been caught off gaurd and agreed to do it, but upon mulling over it, changed his mind, thinking about how hard it was going to be. That could have been what their convo was about.
9) Perhaps Dumbledore didn't tell Harry because he needed to give him a reason to take things into his own hands. If the person Harry was worried about was at Hogwarts, he might not have left for fear of his friends, and I think Dumbledore knew that. This way, Harry will have no problem leaving, not wanting to put anyone in danger, and will go after the Horcruxes on his own. Also, now that Snape has secured his place as Voldemort's most trusted advisor for good, he might even be able to help. Now someone Dumbledore trusts is awaiting Harry at his destination, and he won't be completely without backup, but won't be relying on it either.
10) Now the order won't trust Snape, true, but perhaps Dumbledore has another plan of telling them, or it's supposed to be Snape's incentive for helping Harry. If he somehow saves him, then Harry can vouch that he is on the right side. If Dumbledore had told the order his plan, they would have tried to stop him from completing it. Draco might have even been killed, because they would not be discreet as Dumbledore said he had to prevent that from happening.
And another thing- Harry was under his invisibility cloak, but Snape seemed to get that he knew what had just happened. Did he and Dumbledore plan for Harry to be there? I can't remember if I said this before, but maybe they wanted to give Harry a reason to leave Hogwarts. Maybe they knew that he would never leave if he was worried about his friends and their safety there... I don't know. I can't, for the life of me, reason why Dumbledore wanted him to see this, and why he "looked like he was deciding something" before he said "I trust Severus Snape completely" with no other explanation. I was so angry at Harry for interrupting... I WANT TO KNOW!
Ok, as I said, I did not compile this list of proof, though I did come up with many of these same points upon completing the book, and I stand behind many of them. If anyone would like to argue any of the points, by all means, tell me what you have a problem with!