Joker is a psychological movie, showing the dangers of group action and the power of group narratives.
It is a very interesting commentary on society as it mirrors the phenomenon of deindividuation - where crowds assume a collective identity, dispense of individual responsibility and become willing to commit even the most heinous acts.
Many reviewers see the Joker as a dangerous film because it might inspire incels to identify with the character as a hero and copy him.
The real evil to be feared is a broken, frustrated society that is willing to participate in almost purposeless acts of violence, then put deeper meaning into it, and ultimately use it as a springboard for mass violence and brutality.
Gotham City in Joker is a fundamentally broken city.
Joker is the most realistic depiction of group dynamics. The Joker has no plans, no real motives, no point to make, but is as much a victim of circumstances.
The Joker doesn't manipulate or use other people to achieve his ends. He has no ends to achieve, yet society romanticizes his purposeless actions.
Deindividuation is linked to everything from mass riots to lynch mobs. It shows the dangers of thinking that simple numbers can equate to moral action.
The shared identities of deindividuated groups - where there is a loss of self-awareness - can result in biased recollections and interpretations of events that can create horrendous violence.
We like simple, almost cartoonish, villains that we can point fingers at so we don't have to do any self-reflection or think about our own potential for immoral behavior.
Like the people of Gotham, we want to villainize those who disagree with us while excusing the behavior of our in-groups. This makes mass violence and individuation all the more likely.