I think a lot of people miss the fact that Joker criticises the media's input in society?
-Did you notice that the newspaper painted Arthur as a vigilante against the rich, giving his act a different motive to the one he had? Painting him as an anti-rich symbol was what caused the followers, not the murder.
-Did you notice that the protests were just supposed to be outside the city hall, but turned into riots after the show? Did you notice that the news stations had broadcasted parts that would only fuel the people following this symbol that the media already painted, with his dialogue such as "If it was me dying on the sidewalk you'd walk right over me" and "What do you get when you cross a mentally ill loner with a society that abandons him and treats him like trash?". The riots were caused by fueling the movement even more by broadcasting the guy behind it all, their anti-rich symbol, and his arguments that they would relate with.
If the media didn't cover the subway killings in this way, there wouldn't have been a following. It would just be another tragic murder. The entire film would've gone differently if the media didn't present Arthur as a vigilante, and probably would've just ended with Arthur going ahead with killing himself on the show.
