If your family member was murdered while you were out, do you have the expertise to find out who did it and meter justice?
Do you have the resources to guarantee mutually assured destruction with another community?
If my family was murdered while I was out, then by definition I cannot defend them. The law system will take care of it presumably, but that is not "defense", it is post-fact "justice."
Unclear what your mutually destruction comment was.
Your family is de-facto defended from murder because it is harder to commit murder and get away with it when the FBI exists, generating criminal investigation resources and distributing them among local police detectives. Your taxes pay for this and are protecting you right now. The very existence of a justice system is preventing crime. Not all crime, obviously, but a great deal. The remainder is more a socioeconomic failure of this country than a justice system failure (or, a justice system failure in that the war on drugs is policy failure enforced by the justice system).
Mutually assured destruction is more a macro-comment. Why aren't hordes of Russian barbarians coming over the hill? Because the US exerts its sovereignty within its borders. Why doesn't an organized community (government) challenge that? Because both communities would be vaporized.
Since we're on the subject, your taxes are a much cheaper way to get clean water, medicine that you are guaranteed actually contains the active ingredients you're looking for, food that won't poison you, and ensure that the city upriver doesn't dump toxic waste into the river. If you wanted to "take care of those problems yourself," you'd be running around like a headless chicken. Communitize, instead.