"Sixty nine year-old Ethel Jones poses with her .38 caliber snubnose revolver behind the glass door window she shattered when she fired three shots at a burglar who broke into her Decatur, Alabama home Monday, August 30, 2010. An 18-year-old suspect was taken to Huntsville Hospital with a gunshot wound to the abdomen."
So in this case, Ethel Jones is out a window, but lots of things can break windows. Zooming out, the hospital has to put an 18 year old man's stomach back together, and somebody has to pay for that -- can the hospital do that for less cost than the items a single burglar can carry out of a house on their person? That really depends on whether he found her jewelery case.
And now he is to be entered into the criminal justice system. In the case of burglary, applying justice via legal framework may prevent greater ills that would have come from this man later, or maybe he just wanted a couple dvds to hawk, and now his life is very likely non-valuable (i.e. fucked up), and there are no refunds on the social inputs that go into a life. Whether catching a robber in action is really better for society is a crapshoot.
Zooming out further, the counter argument is that without a framework for occasional, inefficiently over-sized punishment falling on individuals who are caught, profoundly more people would commit the crime in question. I don't think that's true, social motivations don't seem to be affected by the deterrent threat of remotely possible punishment that way -- lots of people die in car wrecks but that doesn't decrease the amount of driving that people chose to do much. But it's a valid argument nonetheless, it's an argument that can be made. The point is that it is really just left to opinion, the complexity of factors which go into actually determining as fact whether firearm possession leads to more good or more bad social outcomes is staggering.
I'm not trying to suggest that Ethel should have to have her stuff taken just because there are costs to protecting it. But on the other hand, stealing isn't the same as destruction. Somebody would still have that stuff, and at a reduced price than what they would pay at Target. Imagine then if the social response to robbery was for the neighborhood, town, whatever, to just say, "Aw shit, here's some replacement stuff or gift certificates Ethel".
Broadly speaking, I don't feel safe around civilians who have a gun, but I don't feel profoundly safe around police who have guns either. I do not think possessing a gun makes a person less likely to die by being shot by a gun. I do not think trying to suppress gun ownership can be done without huge problems and costs, but on the other hands there are probably some pretty low-cost ways to discourage gun ownership and criminal use. For example, why do police need to carry guns? What would happen if regular police didn't, instead only SWAT squads or the like? If all beat police were known not to have guns, would any criminal really ever shoot at a police officer, knowing he could just run away instead? Maybe. Ethel shot at somebody she could have just threatened away.