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. You’ll see- I’ll show you. When the chips are down these, uh, civilized people? They’ll eat each other. See I’m not a monster, I’m just ahead of the curve.
Have you seen Mad Max? If not, there is this Post-Apocalyptic society in which there has been a lot of wars and humans have eventually run out of water sources. So, there is a scarcity of water and thus, food. The result is the violent society where there is no civilization. There is no humanity, there are no rules, there is no remorse. There are these violent races and the winner gets it all.
I am not saying that is the inherent nature of humans. But at the same time, it can be. See, in order to understand Joker, you need to roam around in his skin, wear his shoes and think through his brain. Joker was an extremist. He was a nihilist. He was an anarchist. For him, most of the mundane things did not matter at all.
What the Joker means by this is that no matter how civilized we pretend to be, no matter what laws and regulations we create for our society, no matter what religion we place our faith into in the end when the chips are down, we are all just animals living in a dog eat dog world. Self-preservation will always will out as this is the natural law of all living things, not just humans. Satanism is probably the only religion that recognizes and accepts this.