I have explained how the alternative is worse. Why do you disagree? How could society get around the general scenario I described?
Which police forces should be able to arrest people? If my two best friends and I call ourselves police and lock you up for 20 years because we don't like your hair cut, how is it not kidnapping? Which courts get to decide whether people are guilty of crimes? Who decides what the crimes are? What could be done about invasions? What would stop the Russian, Chinese, or British government from becoming the government?