A Case for the Good of Severus Snape
I don't have the time I'd like to put down the major, in-your-face sort of argument here, as I've got a good two hundred pages left in HBP, but the basics:
If you'll remember (ha ha), Albus Dumbledore's last words were "Severus...please...". To anyone who does not know Albus Dumbledore from a turnip, it would seem as though he was begging for his life. However - as early as Sorcerer's Stone, we learned that Dumbledore was not afraid of death when he said: "To the well organized mind, death is but the next great adventure." In addition to this, Dumbledore stressed throughout the series, mostly to Voldemort, that there are fates worse than death. Which leads me to believe that Dumbledore was asking for death, rather than pleading for mercy...
On page 405 of HBP, Harry learned that Hagrid overheard Snape and Dumbledore fighting in the forest, Snape saying that he "didn' wan' ter do it anymore". This fits exceptionally well with the "asking for death" bit, as Snape likely got cold feet and decided he didn't want to kill Dumbledore, while Dumbledore insisted, knowing that without his death, Snape would himself die (because of the Unbreakable Vow made with Narcissa). With Snape gone, the Order would have no inside man posing as a Death Eater, which would be a great loss to their side.
One I was rather proud of finding all on my own last summer, seaching for anything Snape-y during the annual run-through: Near the end of GoF, as Dumbledore, McGonagall and Snape forced their way into "Moody's" office, it mentions all their faces reflected back at them in the Foe Glass on the wall. It explicitly says, a paragraph later, that "Snape followed [Dumbledore], looking into the Foe-Glass, where his own face was still visible, glaring into the room." (679) Jo drops it in quickly, but as she makes specifically sure to show Snape in the Foe-Glass, she makes a subtle but strong point - Snape is a foe of Crouch, Death Eater extraordinaire and proclaimed most faithful servant of Voldemort. Snape is reflected with Dumbledore and McGonagall, two of the most active Order members, both very much against Voldemort.
Pages 709-10 of GoF: Snape marches up to Fudge and reveals the mark on his arm, without any prompting, to try and convince the Minister of Voldemort's return.
Lastly, Dumbledore stresses over and over and over, even to the point of anger: he has complete trust in Severus Snape. When we can't trust Dumbledore, ladies and gentlemen, we are in deep shit.
And yeah, Snape's been a bit of an asshole to Harry and friends throughout, but when his general character (all sex and glares) and unfortunate past are considered, it's understandable.
Plus, how infuriatingly smug would Harry be? "I knew it all along - right bastard, that one..." Come on now.
I know there's more, but my mind's turning to sludge and I've still got loads to do, so that'll have to do. Trust Snape! He is awesome and sexy and wonderful and GOOD, DAMMIT!
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