It's hard to say exactly. It might result in Adrien getting caught up in a web of deception and danger. Or perhaps he thinks he can change Hawkmoth for the better.
It could lead to all sorts of interesting plot twists. Maybe Adrien has his own reasons for helping Hawkmoth, or it could be a case of being misled.
It's possible that a trusted ally shares vital information with Adrien, enabling him to unmask Hawkmoth. Or, maybe he catches Hawkmoth in the act during a critical moment.
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.
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.
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.
It might be that a trusted ally shares some vital information with Adrien, allowing him to piece together the mystery of Hawkmoth's identity.
Well, it could lead to all sorts of dramatic and unexpected turns. Maybe Commissioner Gordon has a hard time coming to terms with it or uses the knowledge to protect Batman in a new way.
Maybe through some clues and his own detective work. He could have pieced things together after observing certain patterns or actions.
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.
He might start by taking over some of his father's business tasks, like handling paperwork or scheduling meetings.
In the Supernatural fanfiction, if Mary finds out about Jess, she might be quite shocked. She could initially be confused as Jess is Dean's girlfriend and Mary has her own complex backstory. She may try to understand the relationship and how Jess fits into Dean's life, given her own experiences with love and loss in the Supernatural universe.