Most of the violent acts committed are over things directly related to their illegal activity, (turf wars, and keeping "order" and control) if the activity is legal their criminal activity level would drop. Al Capone was a bad guy and prone to violence, but he mostly killed because of outside factors such as his rivals, and his desire to expand his turf. Without the alcohol war he would not have actually committed most of his crimes.
Outside of prohibition (drugs or alcohol) what has had such a high violence rate? I am guessing nothing.