Snape had no misgivings about working for Voldemort at the time he overheard the prophecy, yet a short time afterwards he changed sides and turned spy for the Order of the Phoenix. Betrayal of Voldemort is not a step that could be taken lightly. What caused this complete change of heart?
It was only once Snape realised how Voldemort interpreted the prophecy—deciding to pursue James and Lily Potter—that things changed completely. We can be confident that it was not James’s death that Snape feared since there is overwhelming evidence in the books of Snape’s hatred of James, stemming from their mutual loathing at school. But if it wasn’t danger to James that motivated Snape’s change of allegiance, there is only one other person for whom Snape could have been concerned . . . Snape loved Lily!
In Harry Potter and the Half-Blood Prince, after Professor Trelawney has unwittingly revealed that it was Snape who overheard the prediction about Harry and Voldemort, Dumbledore explains to Harry how Snape was affected by Voldemort’s interpretation of the prophecy:
“Professor Snape made a terrible mistake. He was still in Lord Voldemort’s employ on the night he heard the first half of Professor Trelawney’s prophecy. Naturally, he hastened to tell his master what he had heard, for it concerned his master most deeply. But he did not know—he had no possible way of knowing—which boy Voldemort would hunt from then onwards, or that the parents he would destroy in his murderous quest were people that Professor Snape knew, that they were your mother and father—
. . .
“You have no idea of the remorse Professor Snape felt when he realised how Lord Voldemort had interpreted the prophecy, Harry. I believe it to be the greatest regret of his life and the reason that he returned . . . .”
This passage makes clear that the only possible explanation for Snape’s remorse was learning of the identity of the people concerned in the prophecy. As we know how Snape felt about James, and that Snape would not likely be sorry to see James die, we must look for information that might indicate Snape’s feelings for Lily. The canon (deliberately I am sure) gives us little knowledge of how Snape felt about Lily, but there are clues here and there.
We know from Professor Slughorn (HBP4) that Lily was a very bright, vivacious and charming girl. J. K. Rowling is also quoted as saying of Lily “She was like Ginny, she was a popular girl. She was a bit of a catch” (TLC, pt. III). So it is not unreasonable to postulate that Snape might be one of Lily’s many admirers. And, as Slughorn says in HBP9, the power of obsessive love is not to be underestimated.
In the only mention of Snape and Lily together (OP28), Lily is being sympathetic to Snape as she tries to prevent James taunting him. Although Snape calls Lily a Mudblood, this may be explained by the public humiliation he was being subjected to. We all lash out in such situations and say things we do not mean. Alternatively, Snape may be concealing his feelings toward Lily from his classmates. At the same time Lily expresses contempt for James, which may further endear her to Snape.
Following his explanation of Snape’s mistake Dumbledore “looked as though he was trying to make up his mind about something. At last he said, ‘I am sure. I trust Severus Snape completely’” (HBP25). Dumbledore was clearly considering whether to tell Harry something more—namely to tell him of Snape’s feelings towards Lily, but decides against it as the time is not right.
Snape has always loathed Harry, comparing Harry to the hated James. But Snape’s loathing for Harry could be twofold: Harry not only looks and acts like his father James, but has his mother’s eyes, surely giving Snape a continual burning reminder both of Lily’s untimely death and that it was the hated James whom she loved.
Finally, Snape’s love of Lily might also explain an intriguing question I have with the canon. When Voldemort kills Harry’s parents, James’s death is different from Lily’s. Voldemort was ultimately after Harry that night in Godric’s Hollow, and both James and Lily were there to protect their son. One would expect Voldemort to kill both James and Lily—why then did Lily have a choice while James did not? A possible answer is that Voldemort gave Lily the choice because he knew of Snape’s desire for her—that is, Voldemort intended Lily’s life to be Snape’s reward for the information he had provided on the prophecy. Further, shortly after the night of the murders, Dumbledore knows explicitly that it was Lily’s sacrifice that saved Harry, rather than James’s death. Dumbledore uses this knowledge as the basis for Harry’s future protection by making Harry’s home where his mother’s blood dwells. How does Dumbledore know that Lily was given a choice? It seems probable that Snape, as the only member of the Order of the Phoenix who ever worked for Voldemort, was the source of this information.
The hypothesis that Snape loved Lily explains both why Snape would regret his actions when, and only when, he discovers to whom the prophecy applies, and also why Voldemort offers Lily, but not James, the choice to live. Further, it is Snape who, on realising the threat to Lily, informs Dumbledore that the Potters are in danger. Following Lily’s murder Snape changes his loyalty from Voldemort to Dumbledore, not because he has renounced the Dark Arts but in a quest for revenge for the murder of the woman he loved—a very strong motivation for a change of allegiance.