The movie is dark. I got to read the script. It is REALLY dark. Also, the best thing you can make to a villain is make the viewer understand why he is like that and his views. Joker does that so perfect... You truly understand why Arthur does what he does, even though it is terrible, the movie makes you feel like everyone out there DESERVES what they get, and even ask yourself if you would do the same when so MANY bad things happened to you and you can do nothing to stop it until you just... break. So, from a point of view, this movie makes you feel, understad, and even like what the Joker does and feels about everyone. Of course, this doesn't mean you are a psycopath, it just means the movie is great and knows how to make a great character. Many great pieces of literature did the same (Watchmen as a good example).
What is scary is that there may be some idiot who doesn't understand it isn't real, that the movie doesn't want to make the Joker a good guy, and maybe even tries to do the same thing. But that's not the movie's fault imo. There will be backlash, i am sure of that, but not because the movie is bad. I am even more sure of that.