I don't think politicians should take aim at any form of art just because it contains violent content. Art is provocative, it's a expression of the human experience, and many times that experience is ugly, tragic, brutal, and unjust. That isn't something we should just shy away from because we wish we lived in a better world where these things didn't happen.
I agree and you can easily say this is illegal but the appeal is still there. Maybe even more so for those on the fringes. You can go the China route and ban anything seen as harmful or do nothing which could inspire the next generation of whatever is being promoted. I think the solution is somewhere in the middle. Do enough research to draw a correlation between consumption violent media and behavior. If there is one tax the producers like cigarettes. If there isn't then leave it be.
I only say that because even though games are art they are also products first and foremost. I don't put them on the same level as a shocking image from a war photographer or non-fiction book about a tragic event. Modern music and film are the same way to lesser extent. Hopefully you can take the extra revenues and put in place helpful resources for those going through what some pieces of art seek to glorify.
Haha, got on a bit of a soapbox. My apologies.
P.S. context is also important. Obvious works of fiction or historical dramatizations are different than a contemporary artist talking, singing, writing about illegal/violent acts they may or may not do.