The true allegiance of Professor Snape has been one of the great mysteries of the whole series: is he evil or just greasy? At this point, the volume of evidence stacked against him is overwhelming. He was formerly an evil Death Eater. He was the head of the evil Slytherin household. He has evil black hair. He is played by actor Alan Rickman, who is himself evil. This is some pretty damning stuff.
On the other hand, Dumbledore trusted him, and Dumbledore seemed to know most things about most things. Dumbledore was pretty old though, and old people do crazy things all the time. I once had an elderly relative who stood up and started applauding at the television. He was watching The Muppet Show. That was pretty weird.
Temporarily setting aside the possibility that Dumbledore was senile, one unavoidable fact remains: Dumbledore was murdered by Snape. That pretty much proves Snape is evil, right?
In the opening passages of The Half Blood Prince, Snape agrees to an Unbreakable Vow to "aid Draco on his mission, and complete it should he fail." A careful reading of this chapter reveals that Snape may have been fishing for information during this whole passage, possibly acting in his role as a spy for Dumbledore. As such, he may not have fully known what Draco's mission was when he was trapped into making the Unbreakable Vow. Thus when he smote Dumbledore off the balcony that fateful evening, he may have literally had no choice.
Or, and we're not discounting this, maybe he's just a huge evil dick.
Our "100% Lock, Money-in-the-Bank" Prediction: Occam's Razor suggests that the simplest explanation-Snape is evil-is likely the correct one. But for Snape to be truly evil, that means that Dumbledore would have been wrong about something. And until we see proof of that, we're going to stick with our needlessly convoluted conspiracy theory. In other words, fuck you, Occam's razor. You're one smug principle we can live without.