The constitutional right to bear arms is historical, predating the armed forces and police forces now in place in the US. It has nothing to do with gangs, armed or otherwise. Most Americans who insist on their right to bear arms don’t do it for the original intention behind the constitutional right. They also seem to have little concept of basic gun safety, hence some of the dreadful accidental shootings that have taken place.
As for the Columbine shooters, Dylan Roof and their ilk, the chances are that they would have still found a way to be armed in some way and committed an atrocity regardless of gun control.