In this fanfic, it could be that Ron has been possessed by a malevolent entity. This entity wants to use Ron to get to Hermione for some reason. Severus, with his skills in the dark arts as well as defense against them, detects the possession and saves Hermione. He then works on exorcising the entity from Ron.
One possible plot could be that Ron is under the influence of a dark spell that makes him act aggressively towards Hermione. Severus, being observant, notices this and steps in to protect Hermione. He uses his potions knowledge to create an antidote for the spell on Ron.
Severus might be developed as a more complex and empathetic character. By saving Hermione from Ron, he shows a side of himself that is protective and caring, which is different from his usual cold exterior. This could be a turning point in how the other characters view him.
Another way Severus could save Hermione is by using his knowledge of the Dark Arts in a positive way. If she was hurt by a Dark spell from Ron (in the context of the fanfic), Severus might be able to counteract the spell's effects. He has extensive experience with Dark Arts spells, so he could figure out what spell Ron used and then use a counter - spell to reverse the damage.
There might be some kind of misunderstanding. For example, Ron could have been misinformed about Hermione's actions. He thought she betrayed him in some way, like sharing Gryffindor's secrets with Slytherins, so he lashes out at her.
A more complex plot development might involve the origin of the snake. It could be a dark wizard's pet that was set loose deliberately to target Severus. After Hermione saves him, they could team up to find out who was behind it. This would lead them on a journey where they have to use both of their skills. Hermione's book - knowledge and Severus' experience in the dark arts. Along the way, they might discover secrets about each other and their families that were previously unknown, which could further deepen their relationship, whether it becomes a friendship or something more ambiguous.
One possible scenario could be that Ron says something really mean to Hermione during an argument, and Draco, who has secretly been observing Hermione's intelligence and kindness, steps in to defend her. Maybe he uses his quick wit to shut Ron down verbally.
In a fanfic with this theme, perhaps Hermione discovers that Ron has some habits or values that she can't tolerate in the long run. For instance, his lack of punctuality or his laid - back attitude towards certain important matters. She might start to compare her relationship with Ron to what she could have had with someone else, like Viktor Krum. She may regret choosing Ron when she sees how much more in - sync she could have been with someone who shares her passion for knowledge and self - improvement on a more consistent level. It could also be that Ron's family dynamics start to weigh on Hermione more as time passes. The chaos and sometimes backward - thinking in the Weasley family might make her feel out of place and question her choice to marry into it.
Well, it could be related to Ron's insecurities. He might be insecure about his place in the group with Harry and Hermione. Hermione is always so confident and sure of herself, and Ron might feel that he doesn't measure up. This could translate into aggression as a way of lashing out. Also, perhaps there's some external influence, like a dark magic object that's affecting Ron's mood and behavior towards Hermione.
One possible plotline could be that Lockhart attempts to use Harry for some self - promoting stunt that goes horribly wrong, and Severus steps in to rescue Harry from the immediate danger. For example, Lockhart might try to make Harry participate in a dangerous magic display for his own glory, but Severus, seeing the real danger, intervenes.
In the fanfic, perhaps there was some deep - seated jealousy in Ron. He might have felt overshadowed by Hermione's intelligence and Harry's fame for a long time, and it all just boiled over in this really wrong way.
One possible motivation could be that Draco has changed. He might have seen the error of his ways and decided to do the right thing. Another motivation could be that he has feelings for Hermione, even if they are not fully realized yet. He doesn't want to see her in a bad situation, so he steps in.