It's been said before (and I tend to agree) that it is more heroic to turn away because you've realised you've joined nazis with superpowers (not my words btw) than because they're threatening a girl you want. We are never shown in the text that Snape ever really disagreed with their views on what should be done with muggles and muggle-borns, or if he was just doing it becuase they killed Lily.
My reading of the text was that Lily was the catalyst for a much deeper realization of what he'd gotten himself into. We are never shown that he ever really agreed with the DE views on muggles and muggle-borns. What we are shown is that Severus tended to turn a blind eye towards anything that he didn't really want to see: He didn't want to see that Lily was starting to "fancy" James, and so he didn't hear any of her critical remarks directed towards Severus, only those directed towards James. He was so invested in the idea that the Death Eaters offered prestige, power, glory, and advancement in wizarding circles that he mentally "filtered" anything to do with their darker side; I really think he had himself convinced that, "Oh, it's not that bad, it's just something they say, you don't understand, I'll be so impressive a wizard once I'm a real Death Eater!"
The text also strongly implies that Severus' duties as a new Death Eater were pretty much restricted to spying. Fanon notwithstanding, it's highly unlikely he ever had to do an "initiatory" killing and/or torture, because his conversation with Dumbledore in Harry's sixth year indicates that he's only had to watch people being killed (and I wonder if that was in his early years, or only after Voldemort's return), never had to do any killing himself. That being the case, he was probably somewhat sheltered from the harsh realities of what the Death Eaters did: He focused on spying, something he did well and prided himself on, and was likely all caught up in how awesomely well he would PROVE himself and finally gain recognition! --until the target of one of his intelligence reports ended up being Lily.
So I think he left because the ugly realities of Death Eaterdom hit home for him, became "real" when it involved someone he knew and still cared about. (Not so sure about the "loved her forever" bit, Harry's immediate storyspinning notwithstanding, but he obviously didn't want to see her dead, either.) It brought the killing and cruelty out of an abstraction in his head into a flesh-and-blood reality.
no subject
My reading of the text was that Lily was the catalyst for a much deeper realization of what he'd gotten himself into. We are never shown that he ever really agreed with the DE views on muggles and muggle-borns. What we are shown is that Severus tended to turn a blind eye towards anything that he didn't really want to see: He didn't want to see that Lily was starting to "fancy" James, and so he didn't hear any of her critical remarks directed towards Severus, only those directed towards James. He was so invested in the idea that the Death Eaters offered prestige, power, glory, and advancement in wizarding circles that he mentally "filtered" anything to do with their darker side; I really think he had himself convinced that, "Oh, it's not that bad, it's just something they say, you don't understand, I'll be so impressive a wizard once I'm a real Death Eater!"
The text also strongly implies that Severus' duties as a new Death Eater were pretty much restricted to spying. Fanon notwithstanding, it's highly unlikely he ever had to do an "initiatory" killing and/or torture, because his conversation with Dumbledore in Harry's sixth year indicates that he's only had to watch people being killed (and I wonder if that was in his early years, or only after Voldemort's return), never had to do any killing himself. That being the case, he was probably somewhat sheltered from the harsh realities of what the Death Eaters did: He focused on spying, something he did well and prided himself on, and was likely all caught up in how awesomely well he would PROVE himself and finally gain recognition! --until the target of one of his intelligence reports ended up being Lily.
So I think he left because the ugly realities of Death Eaterdom hit home for him, became "real" when it involved someone he knew and still cared about. (Not so sure about the "loved her forever" bit, Harry's immediate storyspinning notwithstanding, but he obviously didn't want to see her dead, either.) It brought the killing and cruelty out of an abstraction in his head into a flesh-and-blood reality.