I like to cite to Adam Smith's comment in wealth of nations that:
This distinction being well understood, the history of all ages, it will be found, bears testimony to the irresistible superiority which a well-regulated standing army has over a militia.This could would be scary if the meaning were "well-trained" rather than a standing army firmly under civilian control since the fear was that a well-trained, well-armed, professional military could indeed overrun a poorly trained civilian force.
Additionally, the Constitution was partially a response to Shays Rebellion, which was an out of control mob: not a militia. Shays Rebellion was firmly in the minds of the people who were debating the Constitution and its ratification. They would not have wanted a militia which was not firmly under control.
As I have said before, the issue isn't personal arms in the Constitutional debates as much as it is the nature of the defence establishment and civilian control over the military. To say that the term "well-regulated" does not refer to making sure that the militia is firmly under civilian control is to show a degree of historical ignorance which is staggering
Where this falls into the topic of a militarised police force isn't so much that a police force is like Scotland Yard or the French Gendarmerie Nationale as much as that it is firmly under civilian control and well-regulated by rules and procedures which protect the people.
A strong, independent judiciary is a necessary institution in such a society to make sure that the military/police are kept under control. That was why the Constitution specifically sets limits for the Judiciary and guarantees protections in the legal process, both civil and criminal.
As another commenter said, "well-regulated means exactly that", whether one is talking about militias or professional military.
 Adam Smith, Wealth of Nations, Book V, Chapter I-Of the Expences of the Sovereign or CommonwealthPART I Of the Expence of Defence V.1.27