They say liberals and SJWs won't like it, how come?
It's a white guy anti-hero, who actually winds up standing up for himself and others in a twisted way. You can't have that anymore.
It actually works better as a social justice film. The whole film is about a sickly boy, who grows up to be a sickly man, beaten down by a social system that cares more about profit than actually making people well. Most of the victims in the film are rich white men or authority figures.
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One of the major relationships is definitely in his head, and when he figures it out, you'll see more of a change in him. I don't think it was all imagination, but there were certainly delusions.
This is another area where you'll see the "Taxi Driver" influence. There is a faction of film buffs that think Bickle was having one big delusion, and the story never happened. There are also those that wonder "What if Bickle took a more violent turn?" or if Bickle would be content with the cool hero status he receives at the end of the film or would he mentally break down further? Well the Joker sorta runs with some of those themes, albeit in a more fantastical setting.
It's a good film.