Brilliant. Sin City had some of the most amazing imagery I've ever seen in a movie. It was chock full of violence, murder, blood (red, white, and yellow), graphic castration, dismemberment, car chasing, cannibalism, sexism, amputation, chain smoking, irreligion, gunplay, swordplay, nudity, corruption, decapitation, revenge, insanity, double-crossings, suicide, prostitution (both adult and teen), blasphemy, torture, pedophilia, and just for added fun, a case of heart arrhythmia. Every single character was depraved and sickening in one way or another, but then, that was the point. And it was beautiful.
I think the moral of the story can be found when Mickey Rourke's Marv, somewhere in the middle of what will amount to dozens tortured and dead at his hands for revenge over a dead prostitute, merely punches a guard dog rather than killing it. He tells the unconscious animal that he has no argument with it, only with the people it guards. And that is the essence of this movie, that humans are the only creatures with the ability to choose what we make of our lives, and every choice we make will resonate with ourselves and with everyone else. We are responsible for the choices we make. We choose to live in the world we live in, and there can always be another choice.