Earlier quoted context omitted.
Libertarianism != anarchism. It does NOT reject the state's monopoly on violence. It argues it should be used sparingly https://www.theatlantic.com/politics/archive/2016/06/enforci...
To clarify, from my understanding, in a libertarian society, individuals and groups would be able to hire armed mercenary groups a la private security forces, but with presumably less regulation. That's what I mean by "monopoly on violence", eg only the ruling government has the right to use violence to enforce contracts.
Most libertarians want the government to retain a monopoly on violence so that the institution that uses violence is accountable to the public.