Police, at all times, should maintain a relationship with the public that gives reality to the historic tradition that the police are the public and the public are the police; the police being only members of the public who are paid to give full-time attention to duties which are incumbent on every citizen in the interests of community welfare and existence.--Sir Robert Peel, founder of London's Metropolitan Police Force, generally acknowledged to be the first "modern" police force.
There is no moral distinction, in the abstract, between taking violent action in self-defense or in defense of others, and expecting the police to do so on your behalf. There is considerable moral cowardice in believing that you must not act violently in self-defense or in defense of others, and the police must act violently to keep you and yours safe. And there is great hazard in the unsupportable belief that the police will always keep you safe, and that you have no responsibility for securing your own safety or the safety of those around you.
(Hat tip: Kevin at Smallest Minority.)