Critics are afraid of this movie because as far as I'm concerned it is about how disgusting the balance in power in society actually is. It's scaring them because this movie seems far too close to the real world. It is seriously relatable if you have ever dealt with bullying, random acts of violence, being told you were "too weird" and hurt because of who you are. This movie answers the question you keep asking yourself over and over in your head when you are dealing with awful, deplorable human beings who humiliate and hurt you like it's their reason for being. "What if I just hurt them real bad. They deserve it and then they would stop." The thought you never act in but always relish the idea of. How the rich never even have to experience what life is like in the gutter. They never have to ask where their next meal is coming from, they never have to worry if they can afford their house this month, and this movie is like watching a mirror of current society.
This movie could easily be not called or even about the joker. It is a cinematic wonder that stands on its own two feet. Do yourself a favour and watch it.