Boundaries. The Joker violates personal boundaries, that's what makes him a 'joker.'
In the DC Universe the Joker is the super-villain of the Batman story in Gotham City (New York City, though the dark knight trilogy was filmed in Chicago).
Here's the thing, arguably the Joker could also be an anti-hero. Much like Thanos in the Marvel Universe, the Joker is not wrong about anything he says. Technically, his criticisms of society are correct.
But the way in which Thanos and the Joker conduct themselves turns them from anti-hero to villain, they purposefully violate peoples' boundaries in severe and abusive ways. Thanos kills half the world's population to save the planet -- something which, at first, I agreed with. But that was before I learned about personal boundaries, and abuse.
The fundamental tenet of abuse is to force people into situations where 1. they cannot escape, and 2. repeatedly violate their personal boundaries over and over again.
It's wrong because Joker has no regard for personal boundaries of anyone, including himself. That's what makes him a joker. That's where the name comes from.
In both the dark knight and the more recent stand alone release the Joker commits redistributive vigilante justice, killing random people, stealing from gangs, burning money (like burning a pile of toilet paper in 2020) and things like this.