Marronnier

NB: I’m approaching this with my interpretation of book canon, where I got the impression James is not a great person and we don’t see any direct evidence of his having changed after leaving school besides people saying “He’s changed”. Certainly he has some red flags for abusive behaviour, for the purpose of this meta I’m just addressing why she might have chosen to marry him and not trying to say how James was as a husband either way. He might have been a great guy, who knows. Ultimately I’m asking why choose to forgive Snape for the choices he made at 18 and not Lily, when those choices have very similar driving forces.

Timing We don’t know exactly when Snape takes the Dark Mark but probably sometime soon after leaving school. However the foundation building towards this decision certainly happened while still at school.

Similarly Lily marries James probably the year after leaving school, having finally agreed to date him in seventh year. We actually don’t know if they got married before having Harry. Certainly sometime within Harry’s first 8 months - 1 year of life at the very latest, since there are wedding pictures of a big party that would have happened before they went into hiding. But considering the conservative nature of the wizarding world, and the probable attitudes of both their parents, its more likely they got married either the year after school or early in her pregnancy (before showing) at the latest. 

Foreknowledge Would their experiences at school be indicative of life after leaving?

Was Lily misled about the worst parts of James? Yes, canonically. (Lupin and Sirius both admit this to Harry in OotP.) James was on his best behaviour during their seventh year, and they were still chaperoned at Hogwarts under the eyes of the teachers and all their friends, with separate dorm rooms. 

Was Snape misled about the worst of Voldemort? Likely, at least to some degree. Sirius says his own family didn’t really see the full picture until it was too late, including Regulus, which seems to imply there was some level of obfuscation. Certainly the DEs would have on some level tailored their approach to recruiting to what these vulnerable young people wanted to hear. And Snape’s motivations would have been different to a blood supremacist like Regulus yet they both joined up around the same time.

Was it to their benefit not to dig too deeply into any doubts or fears they may have had about James/Voldemort? Absolutely, yes, in order to keep going with their choices.

Background Both Lily and Snape came from a “less than” background compared to their peers in the wizarding world:

Class/Wealth We know in detail about Snape’s disadvantages. I think there’s a misconception that because Lily did not come from the same level of abject poverty as Snape did, that she was in a much higher class than he was. But their worlds were close enough that they met on the same playground, and Petunia knew of Snape’s family. The distinction Petunia makes between them isn’t the exact same as the one James makes.

Blood Status Lily was Muggleborn in an increasingly intensifying wizarding war where her blood status made her place very precarious. Snape was a half blood with one Muggle parent which in this blood quantum system seems to place him only one step above Muggleborn.

Merit I think sometimes it’s easy to conflate Lily with someone like Hermione since they’re the characters both singled out for being Muggleborn as well as studious and high achieving. But Hermione’s parents are well off (no issues shopping for supplies, buys Crookshanks, ski vacation, etc). Lily has a far more precarious situation. She seems to have both worked hard and had natural aptitude, was a prefect and Head Girl, member of the Slug Club. She was working very hard to make a place for herself in the wizarding world and yet I think both she and Snape were astute enough to recognize that the WW is not a meritocracy (I have read some great metas on this), especially by the end of their tenure at school. Snape went to Hogwarts thinking one thing and would have been disabused of this notion soon enough. He doesn’t appear to have striven for public recognition the way Lily did, maybe believing he would need to rely on himself alone to get ahead, but I think this is a parallel where they both realize they need to do something beyond being smart and talented alone.

Familial Support The wizarding world appears to be fairly conservative when it comes to families. Women always take their husband’s last name, at least in the Hogwarts class that we’re privy to. They have children quite young. Outsize importance is placed on family name and dynasty. Neither Lily or Severus has familial support within the wizarding world. And no family is a very lonely place to be when you’re just becoming an adult.

It had to be very tempting to be offered an instant, rock solid place in the wizarding world. A pureblood name, a family, status. A place in the DE, even at a very low level, maybe a promise that merit would mean something there.

Could they both have made a different decision, yes, of course, they could have gone out into the unknown world without protection and rely on themselves alone, but that doesn’t sound very appealing as an adult and certainly not to someone barely past childhood.

From the prevailing attitude at the time (widespread fear among even established adults and families) it seems like they might both view the chance of being a casualty of the war as pretty high for either of them without making the choice they did. They’re both looking for protection, belonging, certainty in an uncertain time. Both are trying to cement their place in the world, not slip through the cracks. 

To me, I have sympathy for someone as disadvantaged as Snape joining the Death Eaters regardless of his later redemption. This eventuality was a result of a lot of factors mostly beyond his control. And I feel the same for Lily. She was doing the best she could with the information she had. We don’t have any sense of her interiority in canon so we don’t know her feelings or thoughts. We don’t know what James said or did to convince her he’d changed. In canon we certainly see he had people willing to stretch the truth or lie for him. And once out of school we don’t see any evidence of other friendships for Lily besides James and his friend group (this itself is a red flag to me but I digress).

The difference between Snape and Lily’s stories is that he’s given a chance to live almost twice as long as she does. She dies at 21, he at 38. So we see what he does with the time given to him, how he redeems the misguided choices he’s made, starting right before the time she dies, motivated in great part by his relationship to her.

But Lily isn’t given this chance. She dies before any story much beyond childhood even starts. So we don’t get to see how her life would have played out. And I am not saying she would have had to make any different decisions later in life to deserve redemption. I’m saying both she and Snape deserve that initial sympathy for making hard choices, maybe wrong choices, when they are still kids. And that the reasons behind their choices show how similar they are, and reinforce why they might have gotten along so well in some ways, and been unable to understand each other in others.