Well, in some fanfictions, Hawkmoth might use false promises. He could promise Cat Noir something like the power to save a loved one in exchange for his help.
Hawkmoth's character development in the 'hawkmoth gets help from a guied fanfiction' could vary. Initially, he's all about his evil plans. But with the guide's help, he could become more empathetic. For instance, the guide might show him the consequences of his actions on the people of Paris. This could make him hesitate in his villainous deeds. He may also start to adapt new skills or knowledge from the guide, which could change his modus operandi. This new influence could make him a more three - dimensional character, with a mix of his original villainous traits and some new elements introduced by the guide.
I'm not sure specifically as I haven't read this particular fanfiction. But the guide could be someone with special powers or knowledge. Maybe Hawkmoth gets help like strategic advice or some form of magical assistance to further his evil plans.
It could be a character from a different dimension or realm. This character might have some sort of connection to the Miraculouses and sees potential in Hawkmoth. They decide to help him, thinking that they can redirect his actions towards a more positive outcome, or at least a less destructive one.
Perhaps Hawkmoth tells Chat Noir because he believes Chat Noir can understand his motives or is trying to manipulate him into giving up the miraculous.
No. In the normal 'Miraculous' series, Cat Noir is a superhero, not a criminal. Fanfiction might deviate from the original, but the general perception of his character is heroic. So, just because it's a fanfiction doesn't mean he's a criminal.