What is the Joker’s motive?

The Joker of the comics is largely driven by ideological and philosophical reasons rather than money or understandable ambitions. Nicholson’s Joker—while motivated by an inflated sense of self, overactive ego, and specialized narcissism—is a gangster at heart.

What was the Joker trying to do?

The Joker — a mysterious man with an unclear backstory, who’s latched himself onto a mob syndicate to fund a series of twisted, grandiose social experiments — wants to prove that the rotten core of Gotham is a product of its citizens. As he says earlier in the film, “their morals, their code, it’s a bad joke.