picture me this: so lily and snape get into this huge fight in like november of fifth year. cause now lily’s a prefect and she’s started to hang out with remus - so snape’s getting all possessive and jealous. she gets mad and screams about how remus, and his friendgroup by extent, are much better friends than she is, and snape says “fine if you like them so much, why don’t you start dating potter and befriending the whole lot of them”
so she DOES just to piss him off, she dates james as a joke but then she actually kind of starts to fall for him. so now they’re dating and like actually kind of in love with each other for a while. but after OWLs like on the last few days of school there’s a big party and snape calls her out for fake dating james. so all the marauders get mad at lily and snape and that’s why sirius tells snape how to get into the whomping willow. the next night Lily sees both snape and james running down to the tree and she knows something bad is happening but not quite what.
so she follows them into the shack and before they can realize what’s happening, remus essentially attacks her. so they drag her back up to the castle but it’s too late, the book ends with her waking up the next morning realizing not only has she subsequently become a werewolf, but she is also completely alone. her boyfriend is mad, her best friends won’t talk to her, and it’s going to be the worst summer ever.