A recent comment on the Snape the Hero post prompted me to respond with the following. Since I think it’s a theory that many people hold, I wanted to address it in it’s own post rather than in the comments.
I think Snape was in love with Lily. Actually, we know they had something in common: Potions. According to Slughorn, Lily was brilliant in it and obviously so was Snape. It’s likely they had that class together, especially since Gryffindor and Slytherin always seem to have Potions together. They probably developed a friendship and then Snape developed feelings for Lily. Unfortunately for him, she fell in love with James. Notice too, that Snape constantly insults Harry’s father, but has never said so much as a word about his mother. This could also be the reason that Dumbledore believed Snape when he turned spy for the Order.
However, I don’t know that I buy the whole Voldemort was going to spare Lily for Snape theory for a few reasons. Number one being that it would be extremely out of character for Voldemort to care in the slightest that one of his Death Eaters had feelings for one of his possible victims. In fact, I’d say that would make Voldemort more likely to kill that person, since he doesn’t want his followers to have an allegiance to anything but him. Number two, how could Snape have been privy to this plan? If he truly loved Lily in anything other than a completely selfish manner than he would know that murdering her husband and child would be devastating to her. How could he want to hurt her that way? Three, by the time the Potters were killed, Snape had already turned spy for the Order. (Dumbledore reveals this to the court in the pensieve in GoF) If he became aware that Voldemort was planning on murdering James and Harry, and knew how to find them, wouldn’t he have tried to prevent it, if he is really good?
To me, the only way that this theory of Voldemort intending to spare Lily for Snape’s sake can only stand up if Snape is truly evil and self-serving. Of course that is a possibility. One would have to be pretty evil (not to mention hopelessly mistaken) to imagine dancing with Lily on the graves of her husband and son. But you still run into the problem of it being unrealistic for Voldemort to care about such a thing as love. In fact, that would be a contradiction of nearly everything we know about Voldemort. He disdains love. He sees it as a weakness. It’s unlikely that he would condone or encourage such a weakness in Snape by sparing Lily for him.
I believe there must be some other reason that Voldemort was planning on sparing Lily’s life. What, I don’t know. My best guess is that it was something to do with magic, but that’s just speculation on my part.