In fanfiction, the scenario of Draco saving Hermione from Viktor can be quite complex. It might be part of a larger plot where Draco is secretly working against some dark forces that Viktor has unknowingly become involved with. Hermione could be a key target, and Draco, having some inside information, decides to protect her. This could lead to a whole new relationship dynamic between Draco and Hermione, as she now sees him in a different light, and Draco starts to question his own allegiances.
Maybe Viktor has been cursed in some way in the fanfiction, and his actions towards Hermione are involuntary. Harry, being observant, picks up on the strange behavior. He may call upon his friends for help, like Ron. Together, they could form a plan. Harry might use his invisibility cloak to get close to Viktor and Hermione without being noticed and then use a powerful stupefy spell to stop Viktor before he can harm Hermione. And then they would figure out how to break the curse on Viktor.
After Harry saves Hermione, their relationship could evolve in a more complex way. There could be a newfound respect between them. Harry might have a sense of pride in being able to protect her, and Hermione may feel a bit more vulnerable around him. This could lead to some awkwardness at first, but as they work through it, they become closer. They might start spending more time together, not just for school or magic - related things, but also just to be in each other's company. Their friendship could become the cornerstone of their lives, with Harry always being there to protect Hermione and Hermione always being there to support Harry with her knowledge and wisdom.
One motivation could be that Draco has developed feelings for Hermione. So, when he sees Viktor as a rival for her attention or a threat to her safety, he takes action to save her.
In the fanfiction, Draco could be an observer. He might not directly interfere with Hermione and Viktor's relationship but instead watch from afar. He could be in a state of self - reflection, comparing their relationship to his own past relationships or his family's view on love and marriage. As he observes, he might learn something about himself and his own capacity for love, and this could lead to his own character growth without directly involving himself in Hermione and Viktor's engagement.
Draco might be motivated by jealousy. He could be jealous of Ron and Hermione's friendship or a possible romantic relationship between them. So he tries to harm Hermione out of spite.
Well, perhaps Snape uses his position as a teacher to step in. He could have been observing the situation secretly and when Draco was about to do something harmful to Hermione, Snape intervened with a firm command or some magic spell to stop Draco in his tracks.
Perhaps Viktor was jealous of Ron and Hermione's relationship. In the heat of that jealousy, he might have overstepped boundaries and put Hermione in a threatening situation. Ron, being protective, wouldn't stand for it and thus saved her. For example, Viktor could have tried to force Hermione to go somewhere with him against her will.
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.