I know what you mean, it is hard to believe after all these years. but then there's proof that's for it and against it, we'll just have to wait for the outcome. Though, it could be that the only reason Snape went along with all the plans was because he wanted to wait until Dumbledore was dead, and he even acted as if he were putting up a fight against Dumbledore when Dumbledore asked him to kill him during the "argument." See, there are SO many possibilities.