Is the culture of the US built on violence?

Yes and No. The US is an incredibly diverse place. Most people are decent caring human beings who love their family and neighbors and want the world to be a better place. There is a history of gun ownership in the US however (goes back to the settlement of a "wild" country and struggles with Native Americans). This tradition sometimes manifests itself in ugly ways as we have seen lately. The country needs to do something desperately about a huge proliferation of guns. Also the inner cities are tough places with high concentration of poor people that have little hope.