The world is violent, and rap, perhaps more than other music, is about what's going in the world around us. But talking about violence and promoting it are very different things. Hip-hop, despite the popularity of the gangsta-rap, has at times promoted pacifism more explicitly than any other genre since the Vietnam War-era.
Rock, blamed for riots and youth crime back in the '50s and '60s, doesn't have anti-street-violence anthems like "Self-Destruction" and "We're All in the Same Gang," or a multitude of songs that decry the easy availability of guns, like Gang Starr's "Tonz 'O' Gunz" or Nas' "I Gave You Power."
Admittedly, there are plenty of songs where rappers boast of murder and videos where they flash guns. But does it even matter? Inner-city violence was rising in the '60s and '70s, before rap. But for the past two decades, as rap has slowly become ubiquitously mainstream, inner-city violence has plummeted, in some places to levels not seen since Elvis' hips were supposedly inciting your grandparents to immorality.