I think the left really missed the boat on this and I'm someone who is hard left. They will often go into attack mode and cancel mode and with this film they simply didn't understand the concept.
They thought this was about the alt right, like propaganda but it couldn't be further from that. In fact, Joker has a dialogue about mental disability and the lack of funding for mental disability and gun control and a host of other topics that the alt right are uncomfortable talking about.
There is a message in here about being nice to people but that's not a message that is either left or right. The truth of the matter is with a lot of these crazy people out here, they do get pushed over the edge. They are already fukked up and their surroundings will add to it.
If there was proper funding for psychiatry and medications and if it wasn't so easy to acquire a handgun much of this might have been avoided.
However, the character is also left up to interpretation because it's a chicken or egg thing. Is Joker this way and it's drawn out of him or is he biding his time to act out his fantasies, that is left open for you to decide.
Phoenix himself said when he read the script there were parts where he feels maybe Arthur is sympathetic but then there are parts where he's absolutely not sympathetic at all... he's scary and unhinged.