Well, it might be that Adrien has misinterpreted some of Marinette's actions. For example, if Marinette has been secretive about certain things in her life, like her Ladybug duties which she can't disclose. To Adrien, this secrecy could seem like she has something to hide related to Hawkmoth. Also, if there are any magical glitches or illusions in the fanfiction world that make it seem like Marinette is associated with Hawkmoth, that could be a factor.
One possible reason could be that Marinette has been acting strangely in ways that seem to Adrien to be related to Hawkmoth. Maybe she has been seen in places or at times that are suspicious, like near akuma attacks or where Hawkmoth might be operating from. Another reason could be that someone has been spreading false rumors about Marinette, leading Adrien to believe she has some connection to Hawkmoth.
One possible reason could be that Adrien is being blackmailed by Hawkmoth. Maybe Hawkmoth has some information or has threatened someone Adrien cares about deeply, like his friends or family. So, in order to protect them, he decides to help Hawkmoth, even though he knows it's wrong.
If Adrien were Hawkmoth in fanfic, it could change the dynamics of the story completely. It might explore his internal conflicts and the impact on his relationships with other characters.
Marinette could first be very confused as to why Adrien would think that. Then she would likely be angry. After all, she is actually fighting against Hawkmoth as Ladybug. She would probably start gathering evidence to prove her innocence, like showing Adrien her normal life activities that are completely opposite to what Hawkmoth stands for.
If Adrien becomes Hawkmoth in fanfiction, Paris would be in for a major shock. His friends, like Nino and Alya, would never suspect him at first. And Marinette, who is Ladybug, would be facing a completely different kind of enemy. She's used to fighting Hawkmoth as this unknown figure, but now it's someone she knows so well. It would change the dynamic of their battles entirely.
Perhaps Adrien becomes Hawkmoth because he's been pushed to the edge by Ladybug's rejections. As Chat Noir, he's constantly in love with her but feels unappreciated. The pain of this unrequited love could twist his perception of good and evil. He might think that if he were in control as Hawkmoth, he could make Ladybug see him in a different light. He could use the power to create situations where Ladybug has to rely on him more, or at least understand his pain.
Sometimes, it's just a different take on their characters for the sake of creating drama. The author wants to explore a new dynamic that goes against the normal love - interest relationship. So they make Adrien and Marinette hate each other to see how the story unfolds in a completely different way from the original plot.
After getting married, they would become more open and trusting with each other. Since they know each other's identities as Ladybug and Chat Noir, there would be no more secrets between them. Their relationship would be more stable and they would support each other even more in their daily lives and superhero duties.
One possible motive could be that in the fanfiction, Adrien is being controlled by some dark magic or an evil force. This external influence makes him act against his true nature and kidnap Marinette. For example, perhaps a villain has cast a spell on him to use him as a tool to get to Marinette, who might have some special power or knowledge that the villain desires.
It could also be due to a spell or an akuma's special ability. In the Miraculous universe, there are many powers that can immobilize characters. For example, if an akuma had the power to paralyze people, Adrien might have been caught in it. And since he's often in the middle of the action, he'd be a prime target. He might be motionless as his body is fighting off the effects of this external force, trying to break free from it so he can transform into Chat Noir and save the day.
One possible scenario could be that Adrien finally gathers the courage during a school event. Maybe it's at a dance where they are sharing a slow dance and he just blurts it out. Another could be that he plans a really elaborate date first, like a picnic in a beautiful park that he knows Marinette loves, and asks her out there. Or perhaps he asks her out after she has helped him with something really important to him, like when she designed a special piece for his father's fashion line and he realizes how amazing she is and that he wants to be more than just friends.