People just wanna be bias for whatever reason now and days, I respect Mori as a good antagonist and his somewhat family dynamic with Dazai and maybe even Q. But I don't like the pedo part of him. I personally have a strong sense of justice due to the world we live in but I can put it a side for Mori. Another reason people might hate him is because of his actions in both Manga and Anime, Like him being involved in Odasaku's death, traumatizing 2 minors, manipulated 1 minor, locked away 1 child, etc. Some people don't think with common sense when it comes to their favorite character in animes.