Without easy access to firearms, criminals are less likely to be brazen and or deadly. When you take that sort of power out of a criminals hands, they aren't as brave as they would otherwise be. DC should keep restricting guns and they will see the crime rate plummet--especially homicides.
No, the handgun ban has not decreased violence and crime in DC, because crime in DC is as problematic as it ever was. Drugs will lead to violence, whether or not there are handguns in Washington DC. Banning something only keeps the law-abiding person from doing it. Criminals have found handguns in DC, despite limitations, and crime has continued.