It's not the monopoly on force, it's the monopoly on the claim to
the legitimate use of violence, from Max Weber:
<https://news.ycombinator.com/item?id=36317559>
There are numerous examples of private corporations or non-governmental actors engaging in violence, with or without state support or sanction. There are the 100 million souls lost, respectively, to the British East India Company's occupation and administration (as a private entity, with military powers) of India, of the transatlantic slave trade by numerous private commercial operators, and of the genocide against the indigenous populations of the Americas, again much by privately-chartered corporations (as the original British colonies were). There are extant mercenary forces such as Constellis (formerly Academi, formerly Xe, formerly Blackwater) in the US, or the Wagner Group presently transacting genocide in Ukraine. There are oil companies who have initiated coups, paramilitary actions, and assassinations throughout the world. There is the Pinkerton Agency, still extant, and with a storied role in violence against labour and civil rights movements. There are railroads, with their own (private) police forces, which are in fact registered as law enforcement despite being nongovernmental.
The truth is that there is no clean distinction between State and Private use of force, lethal or otherwise. What there is in government is, one hopes, legitimacy and accountability to the citizenry rather than to creditors and investors.