Some people might create such fanfiction because they have a different view of the original 'Harry Potter' series. Maybe they don't like the characters, the plot, or the overall world - building. They want to express their dissatisfaction through creating their own stories that go against the general positive perception of 'Harry Potter'.
Well, in some fanfiction, a writer might portray Bill Weasley as 'sucking' perhaps because they want to create a different or more complex character arc for him. Maybe they have a different view of his actions or relationships within the story. It could also be just a creative choice to add drama or conflict.
Maybe the protagonist has a weak personality. For example, being overly indecisive in making choices, which can make the story progress seem dull and frustrating.
These kinds of stories are not only unethical but also can have a negative impact on individuals and society as a whole. We should uphold values like respect, dignity, and positive family values. There are many other positive things in life that are worthy of our attention, like building strong family bonds, promoting community harmony, and engaging in positive social activities.
One reason could be that the anime production might have faced issues like budget constraints or time limits. They may not have been able to continue adapting the entire manga storyline.