No, they aren't. Not any more than public education is necessary. There are other ways to keep good order and deal with anti-social behavior.
The criminal justice system is a self-serving industry. They have an "us vs. them" mentality and attitude. Given a "monopoly on use of force of violence," they tend to abuse it without consequence, or when there is consequence it is throwing a little fish to "them," well after the system is aware that is has a problem.
-- Properly managed with limits and oversights and rights and the whole shebang, they can be a force for good. --
Yep. And they can be an incredibly powerful force for bad, too. Just like any other tool, all depends on the morality and goals of the people wielding the power. Generally, this sort of job is most attractive to the 4% of the population that is sociopath.
Try keeping law and order without an organized police force and you will have rule by psychopaths. Social taboos, cultural norms, religious constraints, and nice intentions mean absolutely nothing to a percentage of the population. Criminals will kill, destroy, and corrupt entire societies. You will, however, get the same situation when police forces are corrupted by power hungry politicians. Unfortunately, some extremely vile, demonic lunatics have gained power in our federal government, and they have no problem abusing the rights of people. The U.S. Department of Justice, the F.B.I., and other agencies have developed the same attitude as the criminal psychopath.
Politics is what is ruining good police forces. Mayors, politicians, and the ruling class have worked hard to coopt the police and turn them into their personal flunkies. Good men and women thus avoid police work while cowards, and dependent, amoral goons are left to enforce the law. Fixing this is not going to be easy, and I no longer believe it can be changed through elections and law.
We have laws now that should protect the people, but when those who are sworn to enforce those laws openly abuse the law laugh in our faces when we complain, there remains little civil recourse. The Founders of our nation would have long ago declared the government illegitimate, which it clearly has become, and those Founders would have also by now have gathered their water cans to engage in watering the “Tree of Liberty.”
We are a people softened by plentiful food, decent housing, affordable toys, and general comfort. I suspect that as a whole we will cower until the end when we one day wake up and notice that once beautiful tree is now dead. May God forgive us for allowing his beautiful gift to wither and die.