I must disagree with those of you who believe that Snape will turn good. He's a complete and total git who completely betrayed the Order. Of course, we could all go on with proof that he will/will not return to the good side and argue, but that's not what this thread is about.
Now, seeing as I just disagreed with the crowd who believes that Snape will turn good, my obvious link to him from LotR will be Saruman and Gollum. Both of them were originally good. Then, they were corrupted by the evil in the Ring and the lust for it. Just like Snape. He was good (inside) in school, until his seventh year when he became a Death Eater. Seeing as he was already deeply immersed in the Dark Arts, it wasn't hard for Voldemort (oh my, I just uttered his name!!
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]) to convince him to join the Dark side. He lusted for more power and he got it. What makes you think that he's willing to give it up? Was Gollum willing to give it up? True, he did for a short time. But then he lusted for it again and betrayed Frodo. And what about Saruman? He was given two chances to change his ways. Did he? Now I leave you to ponder on that.