Well, in some fanfictions, it could be because Snape's actions towards her friends in the original story. Maybe the fanfic writer wants to explore a different side of their relationship where Lily has had enough of Snape's dark arts fascination or his association with the wrong crowd at Hogwarts.
Perhaps in the fanfiction, the writer is exploring a scenario where Lily discovers Snape's true nature which is not as noble as she once thought. He might have some hidden motives or a side to him that is cruel or self - centered, and that makes Lily hate him.
It might be that in the fanfiction, Snape has made some unforgivable mistakes towards Lily. For example, he could have accidentally endangered her or someone she loves while being involved in some dangerous wizarding affairs. So, Lily hates him as a result.
Maybe Lily has a very different view of the characters. She might think the fanfiction misrepresents Hermione in some way that she doesn't like.
One reason could be jealousy. If in the fanfiction Snape is getting more attention or praise from the wizarding world than Lily, she might start to dislike him. Another reason could be that Snape has a different view on blood purity, and Lily strongly disagrees with him, which leads to her hatred.
People are drawn to it because Snape's love for Lily was so deep and unrequited in a way. Fanfiction writers can play with different scenarios, like what if Lily had returned his feelings. It allows for a lot of creativity.
In fanfiction, one reason could be the Marauders' bullying of Snape. They often target him with their pranks, which are sometimes cruel. Lily, being Snape's friend (at least for a while), might despise the Marauders for their treatment of him. Also, the Marauders' cocky attitude might be off - putting to both Lily and Snape.
It's popular because it explores a darker side of a well - known relationship in the Harry Potter universe. Fans are always interested in seeing different perspectives on the characters, and this concept of Lily bullying Snape gives a new angle to their relationship.
In some fanfictions, Snape may not like Lily perhaps because the writer wants to explore an alternative view of their relationship. Maybe the writer is trying to show Snape's bitterness or a different side of his complex personality that could lead to this dislike.
Snape might be initially in shock. He had trusted Lily deeply, so the betrayal would hit him hard. He could become very angry and bitter, perhaps vowing revenge against her. He might also close himself off emotionally, not wanting to trust anyone again.
In such fanfictions, Lily's forgiveness can make Snape seem more redeemable. It softens his otherwise prickly and often dark character. His actions might be seen in a new light, as if he is now more worthy of understanding and compassion because Lily, who he cared about deeply, has forgiven him.
There are versions where Snape reacts defensively. Let's say Lily confronts him about his actions that seem to be harming her or her friends. He would immediately start justifying himself, listing all the reasons why he did what he did, trying to make Lily see his point of view, even if deep down he knows he might be in the wrong.
It would strain their relationship even more. Snape's love for Lily was already on shaky ground due to their different paths. Seeing her with James would make him feel like she has completely chosen the other side, and he might start to resent her a bit.