This film is the antithesis to The Dark Knight

"You see, their morals, their code, it's a bad joke. Dropped at the first sign of trouble. They're only as good as the world allows them to be. I'll show you. When the chips are down, these... these civilized people, they'll eat each other." -Joker, The Dark Knight

Joker's philosophy was "proved" wrong in TDK by people on the two ships going against his expectations and working together in the time of crisis. In a way, TDK argues that people are inherently good and in the times of crisis they'll set aside their differences and all of their best qualities will come out.

But exactly the opposite happens in Joker 2019. Lynch mobs form, innocents die, people start worshiping a clown and it all happens over a bad socio-economic situation made worse by bad-worded comments from an influential person.