Snape might see a bit of himself in Hermione. Maybe he was reminded of his own impulsive actions when he was young. So, he decides to help her as a way of making up for his own past mistakes.
One reason could be Snape's sense of duty. As a teacher at Hogwarts, he has a responsibility to look after the students, even if they attack him. So, he would put aside his personal feelings and help Hermione.
There could also be a more complex plot - related reason. For example, Snape might know that Hermione is crucial to some bigger plan or prophecy. By helping her heal, he is actually safeguarding the future of the wizarding world, even if it means helping someone who attacked him.
Well, in some fanfictions, Snape might use his advanced potions knowledge. He could brew a special healing potion for Hermione. Maybe it's a modified version of a known potion, like the Essence of Murtlap that has strong healing properties.
One possible reason could be a misunderstanding. Maybe Hermione thought Snape was involved in some dark plot against her friends. In the wizarding world, there are often secrets and deceptions, and if she got some false information, it could lead her to take such drastic action.
Maybe Snape is involved in some sort of mystery or danger within Hogwarts that has to do with magic spells. Hermione, with her vast knowledge of spells and her in - depth study of the Hogwarts library, could be a great help to him in deciphering old spells or magical texts related to the situation.
One possible reason could be related to a difficult potion she's trying to brew. Snape is a potions master, and Hermione might be facing an ingredient or a process that she can't figure out on her own. For example, a rare potion that requires precise handling of dangerous ingredients.
Well, Snape might heal Draco because of his own sense of morality. Despite his dark exterior, Snape has a code of ethics. Draco is still a student, and Snape may believe that every student should be given the chance to survive and learn from their mistakes. Additionally, Snape could be acting on behalf of Dumbledore in some way. Dumbledore often had complex plans, and Snape was his agent. Healing Draco could be part of a bigger plan to keep the balance in the wizarding world.
Perhaps she was poisoned. There are many dark potions and substances in the wizarding world. It's possible she accidentally ingested something or was targeted by an enemy. Snape, being a master of potions, would be the most likely person to figure out the antidote and heal her.
Hermione is also very confident in her knowledge. Snape is an expert in Potions, but Hermione is no slouch either. So, when Snape tries to undermine her in Potions class, she might respond with sarcasm. For example, if Snape corrects her in a way that she thinks is wrong, she could say 'Of course, Professor, your infinite wisdom must be right' in a sarcastic tone.
In fanfiction, sometimes authors like to break boundaries and explore 'what - if' scenarios. For the 'Snape cheats on Hermione' idea, it could be a way to create drama and conflict. They might be interested in exploring the emotional turmoil it would cause for Hermione, or how it could change the relationships within the wizarding world they've created in their story. It could also be a way to subvert the typical hero - heroine relationships we see in the original series.
One reason could be Snape's knowledge. Hermione, being a knowledge - seeker herself, might feel safe knowing that Snape has a vast understanding of magic, especially the darker aspects. So, she believes he can keep her safe from any threats related to that knowledge.
Some fanfiction writers might do it for shock value. They want to create a very different and extreme situation compared to the original story to attract readers' attention.