There could be various reasons for such fanfics. Maybe some writers want to explore different character dynamics by making Ruby a controversial figure. However, this goes against the general perception of Ruby as a heroic and likable character in the original RWBY series. It might also be a way for some to create unique and edgy storylines, but these fanfics are likely not in line with the mainstream love for Ruby in the RWBY fandom.
Well, it's hard to specifically point out certain fanfics where Ruby is hated as there are so many in the vast world of RWBY fanfiction. But generally, they could be those where the writer decides to rewrite Ruby's personality in a negative light. This could involve making her overly selfish or cruel, which is a big departure from the kind and determined Ruby we know from the official RWBY. However, these types of fanfics are not very common since most fans adore Ruby.
Some fanfics where Ruby is hated could be the result of the writer's attempt to introduce conflict from an unexpected angle. For example, they might have Ruby make decisions that seem out of character and lead to other characters disliking her. But these are often just creative interpretations that deviate from the usual positive view of Ruby. In the RWBY community, the majority still prefer fanfics that uphold Ruby's positive qualities.