Perhaps Adrien is under some sort of mind - control spell in the fanfiction. It's not unheard of in the Miraculous Ladybug universe. Hawkmoth might have found a way to manipulate Adrien's mind and make him believe that helping him is the right thing to do. This could lead to Adrien unknowingly becoming an accomplice to Hawkmoth's evil plans.
Another reason could be that Adrien has a misguided sense of loyalty. He might think that Hawkmoth has some noble intentions that are just misdirected. He could believe that by helping Hawkmoth, he can steer him towards a more positive path. Adrien is a kind - hearted character, and he might see the potential for good in Hawkmoth where others don't.
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.
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.
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.
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.
One implication could be that it might strain their friendship a bit. If Adrien really believes this in the context of fanfiction, he might start to act differently around Nino.
One common theme is self - discovery. Adrien often discovers new aspects of his powers or his own strength during the fight with Hawkmoth.
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.
Maybe Adrien noticed some strange behavior from Nino that seemed similar to what he imagined Hawkmoth would do in fanfiction.
On a personal level for Adrien, he would be straying from his own moral compass. He has always been a character associated with good and helping others. By helping Hawkmoth, he would be going against his own values, which could lead to internal conflict and a possible downward spiral in his character development. He might start to question his own identity and the choices he has made, and it could take a lot of effort to get back on the right track.
There's also a great one titled 'Adrien's Duel Against Hawkmoth' on Archive of Our Own. In this, Adrien discovers some new powers that help him in his fight against Hawkmoth. It's really exciting as it shows a different side of Adrien's character and his determination to defeat the villain.
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.