I was talking about this with 2 friends: one HP fan and one who was only one the 2nd movie at the time. We were talking about Alan Rickman. Non-HP fan said he's hot, but then HP fan was like, "Yeah, but he kills Dumbledore!" And Non-HP was like, "OMG! WTF?! Why did you tell me that? You just ruined everything!"
We were trying to explain to her that along with Darth Vader being Luke Skywalker's father, Snape killing Dumbledore is like one of the most known spoilers of all time! We thought it was common knowledge but apparently not. :S