Why wouldn't these putative "criminals" destroy an entire society in order to prevent any comeback?? There are certainly people in the world now capable of such acts, and the concept is common enough in fiction through the ages...although it is true usually 1 child gets away....
What is "natural law"?? The law of the jungle? Might makes right?
And asking this is not a pointless question - the question of how disputes are solved has to be intrinsic to any sort of society - it occurs for all forms of life including plants.
Because morality isn't subjective.
The existence of morality might not be subjective, but what constitutes morality certainly is. What is moral for a communist is different for what is moral for an anarchist which is different from what is moral for a fundamentalist Moslem and a fundamentalist jew and a survivalist and a Jehovah Witness and a Roman Catholic and an Animist and a Southern Baptist and an agnostic.
Sure they might agree that "murder" is wrong - but what is murder might be quite different for some of them - there's plenty of historical evidence of religious fundamentalists thinking that killing people of other religions or atheists or pagans is not murder, various people thinking that killing communists is not murder, etc. The concept of some people being "less than human" is a well established behaviour!
My argument, since you seem unable to grasp it, is that your so-called lack of government will, in reality, be nothing of the sort.
The first thing that would happen without your current formal government is that groups of like minded people would band together and agree to be governed by a set of rules that suits them. And they would agree to collectively enforce those rules using some form of coercion and force - ie they would govern themselves - ergo they would have government.
And weaker groups would find themselves threatened by stronger groups if there was conflict - they might give up and become absorbed, or band together as confederations of some sort to resist the stronger ones - but again you get agreed rules of behaviour, you get requirements to contribute to the collective effort, whatever that may be.
This is reality - this is how human society has always been, and short of some spectacular evolution of our species it is going to stay that way for a long time.