Apparently, for enjoying Sin City, according to Josh Chafetz, of Oxblog.

I think what bothered me most was that some people leaving the theater clearly did enjoy the movie. I worry about the state of their souls as individuals, and about the state of a society that produces people so inured to violence and gore.

Okay. In this follow-up post, he responds to the charge of hypocrisy for supporting the Iraqi war while decrying fictionalized violence:

We all accept violence for certain purposes -- criminal punishment is violence directed by the state against an individual, but we pretty much all think that imprisonment of, say, armed robbers is okay. My support for the Iraq war was always premised on the idea that it would do more good than harm -- that it would prevent more violence than it would perpetrate. That was an empirical judgement -- a judgement that Singer and Yglesias may think was incorrect, but that was the judgement nonetheless.

Sin City depicts violence for its own sake. There's no purpose behind the violence -- it is simply presented as entertaining in and of itself. That is what I object to. I wouldn't object to violence in a movie that got at some deeper message through the depictions of violence. That would be violence in the service of a purpose. But the idea that depictions of violence, without more, constitutes entertainment ... well, yes, I find that deeply disturbing, just as I would find it deeply disturbing that someone supported the Iraq war because he liked seeing destruction on the evening news.

Okay.

It's at this point that I'd explain a little thing called "catharsis" to Josh. The revenge tale is as old as storytelling itself.

They aren't just showing random people getting limbs and heads hacked off. Each of the stories depicts villains preying on or oppressing others, and protagonists who mete out justice in grisly extrajudicial fashion.

Seeing a pedophile get his nuts ripped off isn't violence for its own sake. It's violent, to be sure...grisly as hell. But it definitely serves a particular narrative purpose. This wasn't just lining up random people against a wall and showing them getting shot. That would be senseless.

This was a very old, very popular sort of storytelling. And it was done well.