I've got a new theory about Snape. Well, at least new to me since I don't read HP websites.
I'm figuring that Snape was in love with Lily because of the way she stood up for him against James and company, and that's why Dumbledore felt that Snape's regret was genuine. As Dumbledore says, love is the most powerful of magics that even Voldemort cannot withstand or comprehend. And that also somewhat explains Snape's behavior towards Harry.
While Snape was probably in love with Lily, he despised James. The dislike of James shows with his total disrespect and near antipathy towards Harry. But, because of Lily, he actually does quite a bit to protect Harry. He was the one who notified the OotP when the kids went to the Ministry. He protected Harry from Dolores Umbridge whenever he could. And he stopped the Death Eaters from killing Harry. I seriously doubt Voldemort would have cried any tears if a Death Eater brought Harry's lifeless body before him or even took him captive.
I think JKR is trying to say that love is the most powerful thing through how Lily's sacrifice protected him against Voldemort as long as he was living with blood relatives. And Voldemort could not maintain his possession of Harry because of love. Why not have Snape's love of Lily to be what eventually redeems him or perhaps has him sacrifice himself for Harry (my prediction for book 7)?
