Lily/Snape theory:   Now that Book6 has given us more clues, I will be more willing to talk about it because it won't seem like speculations only. If some of you notice that I missed on some details, please tell me! Here goes:

I think that Snape was in love with Lily from afar at school. She didn't seem attached to him at all in Book5. He said she was a Mudblood when she came to his defence, but only to keep his cover safe within the Slytherins. Especially since he must have felt like Lily being a Muggle-born within wizard-children.  Snape had to grow up at Hogwarts, and even before, knowing he was a half-blood.  Hence, I do believe that is one reason why he would have felt for her: she was like him, a social reject in wizarding society.  Also, she was excellent at Potions and very good at many other subjects. For an intellectual swot like Snape, that must have been quite a turn on. I know, I've been there!! ; )

When he noticed he could never have the girl, but even worse, that she had fallen under the charm of the man she had sworn never to fall for, he must have gone berserk or something. I would be so horribly hurt that the one I loved fall for my torturer!! Sniff!  It would be hell and  if I had been brought up with the Dark Arts like Snape was, I would surely think of taking revenge somehow. Or at least, I would hate the 'winner' so much more because I wouldn't think him worthy of the one I loved. Hence, Lily and James being together gave him a hell of a good reason to try to gain what he couldn't another way or take revenge (either one or both): The DE and Voldemort.

However, I can't decide whether Snape still wanted Lily or not by then, or if he only wanted to prove his might?! But one thing is sure: he was a servant of the Dark for a while  because he reported the prophesy.

When he learnt his reporting half of the prophesy to Voldemort had set off a manhunt for James and Lily along with their baby, he regretted his move then I think. Of course he would if his goal was  getting revenge on James only and not Lily. Who would not feel horrible and guilty to throw the one you love in the hands of a maniac?! Or it could also be that  his goal was to eliminate James and somehow convince Lily to go with him instead, so he still wouldn't want Voldemort to kill her.  She was in danger and it was Snape's reporting the prophesy that did this. Oops! Did he also feel sorry for James and Harry? I can't say. However, I do believe he didn't want anything to happen to Lily no matter what.  How could he save Lily then?  I see two ways:

Option1)Snape wanted Lily. A dead Lily would be worst than anything else. That would be when Snape went to Dumbledore. Albus did mention that Snape turned a spy before the downfall of Voldemort after all. In Book6 we discovered that Voldemort had no quells in killing James but didn't want to kill Lily, we may think that Snape had requested that the woman be handed over to him for his loyal service. If Voldemort agreed, then James would be out of the way, the bady dead and Snape would have his prize.   It so happens that Lily refused to stand aside and she died. Now if Snape had joined the DE to become more powerful and attractive in the eyes of Lily, his plan was screwed up to say the least.

Option2) OR if he didn't want Lily for himself,  he would not have wanted her dead nor her family. Snape wanted the three to live. So Snape would also have been the one to warn the Order that the Potters were now in danger.  That means that he went to Dumbledore before Voldemort found their hide out.  Why offer Lily to live then?  Snape had turned a spy by then according to this option.