I agree with you in the realm of reality. But I think thats why they tried to bring the idea that "some people just want to watch the world burn" to the forefront with Joker. If he wanted to redistribute the wealth to have a better society he wouldn't burn it all, or he'd use it to counter anti social program rhetoric or something. Try to make some kind of change, but he doesn't want change, he wants the wealth gap. The bigger the gap, the more desperate the people who he can manipulate into causing more crime and chaos.