It's strange how our morality works. While Yue Buqun is probably one of the most detestable characters, other characters have done worst things.
Being a hypocrite and portraying a saintly image while secretly being evil to the core inherently annoys us I believe --but I'm not really sure if he is more evil than a lot of other characters that we've seen that we judge much less harshly. Cheng Kun in HSDS for example blatantly does the same thing among other heinous crimes, but he is much less hated imo.
We hate him because the world thinks he's a good guy while we know he's not. It riles us up. But in the end, people like Murong Fu and Murong Bo who are willing to destroy whole nations and millions of civilian lives in order to achieve their goal are much more destructive.