"You wanna know how to get Capone? They pull a knife; you pull a gun. He sends one of yours to the hospital, you send one of his to the morgue."
--Sgt Jim Malone (The Untouchables)
Nice chart that demonstrates how prohibition escalates violence in society. An influential group in society views X (e.g., drugs, alcohol, guns) as threatening. This group influences government officials to pass a law banning X.
Because there is no longer a way to legally obtain X, its value grows (ECON 101--supply goes down in face of constant demand, thereby raising price).
Black markets naturally arise with the increase in price, as buyers and sellers see benefits that outweigh the cost of trading illegally. Crime, i.e., illegal trading of X, becomes more organized over time, and the frequency of violent crime in the market increases.
In response, law enforcement develops harsher tactics for combating the crime. Criminals become more violent in response.
And around she goes...