Private security is by far the preferable way to organize a free society, over the jackboot lawless government thugs we have today.
I tend to agree. It's a very delicate issue. No matter how security and rights enforcement is organized, either by a state or by private entities, there is always the danger of organized coercion against private property. The big question is: In what system is it less likely that this is going to happen, or in which system will it happen to a lesser degree?
As we see today, government officers are constantly killing innocent people, putting them in jail for non-crimes and seizing and damaging private property. That itself does not mean that a truely private alternative (in an overall more or less free society) wouldn't do that, or be any better. But there are strong reasons to believe that this alternative may be better.
First of all, private security and rights enforcement wouldn't have the "halo" of being legitimized by the consent of "the people". So if they go ahead and randomly shoot people's dogs, throw their kids in prison for non-violent "crimes" or start to "tax" people, i.e. forcing people to pay for their service, the public would object to their actions. And they might go to a competitor to protect themselves against this violent organization. Which is the second big advantage: Competition in the field of security and rights enforcement would be seen as the norm, as something usefull to preserve any organization from taking over and starting to tramble our rights. While today anybody trying to start competition to the police force, trying to protect people from the unjust use of force by the state would be seen as a terrorist. Not only would the current police force want to destroy this organization (this might also be the case in a private setting), but they would have the public's blessing in doing so.
It's now even the generally accepted theory that the "European Mirracle" (the sudden economic and cultural rise of the continent in medieval times) was only possible because of its patchwork of small, competing principalities and kingdoms. This was the only reason that allowed a culture of private property and capitalism to develop. But still, there was generally a monopoly of force over a given geographic territory. Enabling competition of security agencies also within a given territory, I believe, would result in an even freer society.