Snape took the unbreakable vow with Narcissa. Therefore he HAD TO kill Dumbledore, because Draco couldn't. Now, about taking the vow itself, I think it's perfectly plausable that Dumbledore wanted Snape to kill him rather than expose his loyalties to the Order of the Phoenix. Dumbledore has mentioned on a lot of occasions that there are things much worse than death.
Also, I think Snape will use the pensieve in book 7 so everyone can see Dumbledore say these things in his own words.
The fact that he saved Harry a couple of times, though, is not evidence enough. He explained all these things to Narcissa and Bellatrix in chapter 2. To be QUITE honest, from the readers perspective, Snape could still be a double double tripple spy for either side.
Oh, and Mark Evans was just a kid that used to get beaten up by Dudleys gang. A lot of people thought he was related to Lily Evans, and created some theories about his future role in the books. J.K. Rowling, however, squashed all of them on her website, when she said Mark Evans was no one. Just a very minor character who needed a name.
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The strength of a civilization is not measured by its ability
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