He could be stealing the potion out of envy. If he sees Harry or another student achieving great things with the help of certain potions, he might want to steal one for himself to level the playing field.
One possible scenario in such a fanfic could be that Draco steals a potion to gain an advantage in a wizarding duel. Maybe he's desperate to prove himself against Harry Potter and thinks the potion will give him an edge, like a strength - enhancing or a quick - thinking potion.
In a fanfic, a Harry Potter potion accident could lead to all sorts of strange consequences. For example, the character might end up with unexpected magical abilities or physical transformations. Maybe they accidentally turn into an animal, like Neville did in one of the real books but in a more extreme way. It could also affect their relationships, as others might be scared or fascinated by the new form or power they possess.
One possible motive could be a misguided sense of protection. Dumbledore might think that taking something from Harry, like a powerful magical item, would keep it out of the wrong hands. He may believe that Harry is too young or inexperienced to handle it properly.
In a fanfiction, Dumbledore might steal out of desperation. Suppose there was a situation where a curse was about to be unleashed on Hogwarts, and the only way to stop it was by stealing a particular ingredient for an antidote from a place where it was not supposed to be taken. Dumbledore, seeing no other option, decides to steal it. His motive here is to save the school and its students, even if it means breaking the rules.
One possible motive could be his intense hatred for the wizarding world. Since Harry represents that world to him, he might murder someone he believes is part of the magic that he despises so much. Another motive could be fear. If he feels threatened by a magical being or situation, in his panicked state, he could resort to murder.
One motive could be love. In a romance fanfic, Draco's feelings for Harry might be so strong that he is willing to go against Dumbledore. He doesn't want to see Harry hurt by anyone, including Dumbledore. Another motive could be redemption. Draco might be trying to atone for his past wrongs and sees saving Harry as a way to do that.
Maybe it's a form of power play. By taking Harry's glasses, Draco is asserting his dominance. In the wizarding world, status and power are important, and this could be Draco's way of showing that he can take what he wants from Harry, who is often seen as his rival.
It could be Neville. He is often the underdog in the Harry Potter series. Maybe he discovers the spanking potion while tending to his plants in the greenhouse. He doesn't realize what it is at first and accidentally sets off a chain of events that involve the potion's strange effects on the students and teachers around him. Since Neville has a kind heart, he would then try to find a way to reverse the effects, which could make for an interesting story.
I'm not sure specifically as there could be many different takes in a fanfic with this title. It might involve a made - up story within the Harry Potter universe where a 'lust potion' plays a central role, perhaps causing characters to have unexpected feelings or actions.
One possible motive could be to explore a dark and alternate universe. The writer might want to see how the 'Harry Potter' world would change if such a drastic event occurred. Another reason could be to analyze the characters from a different perspective. By making Harry the destroyer of Dumbledore, it forces readers to re - evaluate both characters.