In some fanfics, new plot elements are introduced. There could be a misunderstanding between Harry and Lupin. For instance, Lupin might be involved with some dark forces without Harry knowing the real intentions behind it. Harry, being the hero, would naturally hate someone he thought was on the wrong side, even if it was a misperception.
Maybe in that fanfic, Lupin did something that Harry considered a betrayal. For example, he might have made a decision that put Harry or his friends in danger without consulting them first.
It could be that the fanfic writer wanted to explore a different dynamic. Perhaps Lupin's werewolf nature was portrayed in a way that made Harry uncomfortable or fearful in this particular story. Like, Lupin might have lost control in a situation where Harry expected him to be in control, and that led to Harry developing a dislike for him.
One great 'Harry Potter - Arsene Lupin' fanfic is 'Lupin's Legacy'. It delves deep into Lupin's past and his relationship with the other characters. It shows how his werewolf nature affected his life at Hogwarts and beyond in a really interesting way.
At first, it would become very strained. Harry might avoid Lupin at all costs. He would no longer trust Lupin and would be suspicious of his every move.
Character development makes it interesting. In a good fanfic, Lupin's complex character is explored further. His struggle with his werewolf identity, his loyalty to his friends, and his role as a teacher all can be delved into more deeply than in the original books.
Maybe he hates it because it often distorts the real story or characters he knows. Fanfics can take the story in wild directions that don't align with his own experiences.
In some fanfics, Harry might be made to hate Gryffindor perhaps because the author wants to explore an alternative personality for him. Maybe they want to show how the Gryffindor house's values, like bravado or recklessness, could be seen in a negative light from Harry's perspective.
Maybe in that fanfic, Harry has had some really bad experiences with magic that made him hate it. For example, he could have seen magic being misused to harm innocent people. Or perhaps he's tired of the chaos and danger that magic always seems to bring into his life in the fanfic.
One possible event could be that Lupin lied to Harry about something important. In the world of Harry Potter, trust is a huge factor. If Lupin deceived Harry, it could easily turn Harry against him. Say, Lupin lied about his whereabouts during a crucial time when something bad happened, and Harry suspected Lupin's involvement.
It could also be that the fanfic might interfere with the way he wants his story to be remembered. His adventures at Hogwarts were real and important to him, and when fanfic changes the narrative, it's like his story is being disrespected. For example, if a fanfic portrays him as cowardly when he was actually brave, that would surely make him dislike it.
One possible reason could be a misunderstanding. Maybe Lupin did something that Harry misinterpreted as betrayal or negligence. For example, if Lupin was forced to keep a secret from Harry for his own safety, but Harry found out in a way that made him feel deceived.
I'm not entirely sure as I haven't read this specific fanfic. But generally, a fanfic with Lupin and Harry might involve some sort of adventure or relationship exploration within the context of the Harry Potter universe. However, the 'spanks' part is a bit strange and might not follow the typical positive themes of the original series.