So How About if People Start Policing the City on Their Own?

So a couple of thugs commit an armed robbery on a convenience store in Reading, PA and are confronted by an armed citizen outside the store who orders them to stop and stay still until the police arrive.  They don't.  Said citizen shoots and kills both robbers.

The families of the criminals are outraged, and want the citizen charged for "taking the law into his own hands."  Says one:

How about if people just start running around here, policing the city on their own? How much worse is it going to get?

I guess he's never heard of Sir Robert Peel and his Nine Principles of Modern Policing, number seven of which is:

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.

How much worse? Wrong question. How much better?