"You can't have virtue without sin. What I'm after is having my characters' virtues defined by how they operate in a very sinful environment. That's how you test people"

In this quote, Frank Miller is basically stating that in order for a character's virtues to genuinely shine, there should be some type of sin or temptation present in their environment. It's simple to be a great person when whatever around you is perfect, however it's much harder when there are negative influences and temptations present. By putting his characters in these gritty, sinful environments, Miller has the ability to showcase their real character and check their willpower. Basically, it's insufficient to merely have characters who are naturally good-- they must be challenged and evaluated in order to truly show their virtues.