Let's introduce some set analysis.
People who play violent games or listen to violent music mostly do not commit crimes.
People who are criminals mostly have experienced violent material.
Criminals are a subset of People who have experienced violent material.
Freedom of Speech does not preclude public safety, sorry. While we all should understand that fictional material is fictional and that we shouldn't do it in real life, fictional material is based off real environments. The rating system tries to address the level of maturity required to understand the concept of fiction. The rating system is not perfect, being based off age ranges. Sometimes parents understand this and evaluate that their child is mature enough, despite a younger age.
Yes, we shouldn't ban anything. No, not everything should be open and free. Yes, we should have rules to help inform people on factors for decision making.
People have the right to decide for themselves, but they're not always the brightest bunch.