Biosocial theories of crime are not solely biological. They do also include a social element. The social element is important in explaining why levels of violence are higher in lower class areas.
Biosocial theories of crime argue that there is a genetic and biological aspect to crime. Some people are simply born with a predisposition to crime just as some people are born shy and others outgoing. But these genetic factors do not completely determine whether a person will become a criminal. Instead, they interact with social factors. This is why crime (to biosocial theorists) is more prevalent in lower class areas. Those areas are populated by people who are less able to rein in any inborn criminal factors that they and their children might have. Parents have less time and money to spend on their children, for example, making it less likely that they can overcome any genetic predispositions their children might have towards crime.
Violence is more prevalent in such areas, then, because poor social conditions make it more likely that genetic predispositions towards crime will actually express themselves.