Drug-related gang violence isn't caused by drug use, it's caused by drug policy. Changing the policy so that it no longer creates ridiculously huge profit opportunities around the drugs in question does make the problems go away.
Trust me, they will find something else to exploit. Kidnapping in Bogota comes to mind. Crime won't go down.
EDIT: removed fragment: "This is extremely well known". I disagree with the parent, but let's keep the debate as civil as possible. My apologies.