What is the theme in Watchmen by Alan Moore?

The theme of evil being punished is at the heart of the Watchmen. The graphic novel's protagonists are all costumed heroes. They're vigilantes who patrol the streets and prevent crime without any special commission from the State. All have flaws and all have their own definition of evil. This brings up more complex themes about what exactly evil is. Is humanity evil? Is the world really, as the Comedian notes, a cosmic joke?