As far as Pinkertons and strikers go, there was violence on both sides. Strikers damaged plenty of property and continually trespassed on company property back in those days. Moreover, I can provide you with plenty of examples where private security did not do that. Case in point, the security company I worked for contracted with business and residential blocks. The only thing we called the police for was disposition of people who had violated property or persons, since in California, a private security agent can't take that person to the county jail.
Blackwater, insofar as they provide executive security, is an exemplar of high-end security services. However, you can't use their government contracts as some sort of example of possible behavior absent a state, since they are literally functionaries of the state.
I provided physical security and parking enforcement in one of the poorest neighborhoods in Orange County. It's not just for the rich, and if you allow for market competition, security services would be much better quality and much cheaper. Look at alarm companies. There's more than a dozen here in Montgomery county alone.