I guess I must not be terribly good at sleuthing things out, because beyond finding out more about the animosity between James and Snape in its early stages, I still dont understand why Snape does the things he does ie stand up for Harry when he needs it, provide potions for Lupin etc. I *do* understand his hatred of Harry though : its amazing the kind of grudges people can pass on to offspring. Experienced that kind of spewing hatred myself. Had the misfortune of crossing paths with a school administrator that had locked horns with my mother once or twice. He transfered that animosity to me and made me pay for it.
I still have to make an effort to forgive him. Completely uncalled for.
With his swoonalicious voice? Oh yes, yes, yes. What's to forgive??
Snape has an interesting internal code of ethics. Most of what he does, IMO, is out of guilt over his past actions as a Death Eater, or some weird sense of... hatred/gratitude/obligation to James. And he's trying to live up to Dumbledore's trust and expectations.
Of course I'm not sure that Dumbledore is the kindly old man we see him as. Some of his actions are definitely manipulative and I no longer trust him. His interests aren't to keep Harry alive; he wants Voldemort dead. I think he might be willing to sacrifice Harry for that.