By titling book six after Snape, who was revealed to be the Half-Blood Prince, Rowling presents Snape as vitally important to the plot of the series, a two-sided man enigmatic enough to warrant a second look.
The inexperienced reader, inflamed with passion, may be tempted at this point to shout out, He killed Dumbledore! But this interpretation lacks depth, neglecting vital clues to Snapes identity. To careful scholars of the Harry Potter debate (i.e. fanatics), the Snape question becomes a fascinating exercise in reading between the lines (or lies, as the case may be) to decode Rowling's clues.
There is certainly a large body of evidence in defense of Severus Snape. The first and most compelling argument is Dumbledores trust of Snape.
Since Dumbledore is the voice of truth and reason throughout the series, Snape's defection would put Dumbledores judgment in question and re-cast him as a tragically naïve mentor. Readers should expect Deathly Hallows to present the powerful story behind Dumbledores unwavering trust in Snape. An emotionally compelling tale might succeed in convincing the reader to trust Snape.
Another defense of Snape is based on a scene at the end of Goblet of Fire, in which Harry is in the office of the man he believes to be Mad-Eye Moody, but who is actually Death-Eater Barty Crouch, Jr. When Harry looks at the Foe-glass, designed to show Crouch his true enemies, he sees Dumbledore, McGonagall, and Severus Snape. If Snape is a true supporter of Lord Voldemort, he would not appear in the foe-glass of a Death-Eater.
Another persuasive argument, found in the article Snape Clues on beyondhogwarts.com, is that Dumbledore planned for and even willed his own demise in order to promote some greater good. When Dumbledore begs Severus, please, at the top of the Hogwarts tower, one wonders whether the ever-fearless Dumbledore could really be begging for his life. It is particularly questionable considering Dumbledore's serene statement in Sorcerer's Stone: [T]o the well-organised mind, death is but the next great adventure (297). Pleading to be spared seems unlikely for a character who has lived such a long, brave life.
There is, however, a persuasive, if a bit far-fetched, case for Snape's identity as an evil character. Developed by B. J. Texan of mugglenet.com, this argument states that Severus Snape is loyal to neither Voldemort nor Dumbledore; rather, Snape pretends to work for the two parties while secretly promoting the interests of a third: himself. Texan argues that Rowling, a lover of medieval literature, based Snape, the Half-Blood Prince, on the character of General Severus in Machiavellis book The Prince.
Machiavelli's Severus takes power by pitting his opponents (also generals) against each other. He works together with General Albinus long enough to destroy General Nigers army, then turns on Albinus. Could this be a parallel of Severus Snape's dealing with Albus Dumbledore and the Dark Lord?
There are quite a few reasons why this theory makes sense. It offers an explanation for Snapes past protection of Harry despite his apparent loathing: Snape is nourishing Harrys powers so that the young prodigy will destroy the Dark Lord for him. This model also reconciles a dilemma about Snapes talent with occlumency. If Dumbledore sends Snape as a double agent to Lord Voldemort, the greatest Legilimens in the world, does this not imply that Dumbledore believes Snape to be an even better Legilimens? Is it possible that Snape uses occlumency to fool even Dumbledore?
Unfortunately, this newly-surfaced argument does not help readers with the bumper-sticker dilemma. Should we so lightly abandon Dumbledores advice to trust SNAPE in order to proclaim to the world that SNAPE is a very bad man? I hope I trust Dumbledore more than that. Unfortunately, it is Rowling, not Dumbledore, whose character we must assess. Like her Half-Blood Prince, Rowling has already proven herself too slippery to be second-guessed.
Really, the safest bet might be to surrender our minds (and our money) to her literary genius: Simply pre-order two copies of the book, and display both bumper-stickers. Otherwise, devise your own: Forget Snape. Im for HARRY.