New government

Since I am scrapping everything I have learned about the justice system and starting from philosophical scratch, I need to determine what the government's role is (this is the format most will follow, with my rationale and assumptions below the answer in case people don't feel like reading them).

Question:          What is a government's reason for existence and what should be the rationale behind any law passed?

Answer:          A government exists for the purpose of maintaining social order, as such all laws passed by a government should serve to maintain stability in some way.

Rationale:          People, in groups less than 150 people, require no enforcement of moral guidelines (no one does anything immoral because it will definitely come back to be their bane), but when populations rise they require religion and government to enforce those moral guidelines. Since morality has been developed by natural selection as a matter of maintaining group cohesion, morality's purpose is stability. If morality's purpose is stability, and the government's purpose is to enforce morality, then the phrase can be simplified to say: the government's purpose is to maintain social stability and order.

Assumptions:          Michael Shermer (from whose book I got quite a few ideas and pieces of information) had no reason to be deceptive in writing his book The Science of Good and Evil.

Since this justice system is supposed to be a completely rational process, there must also be a rational reason for the punishment of criminals. What I mean by punishment is when the government deliberately and directly reprimands someone for their activities. That definition is such that exile is not really a mode of punishment (nor a terribly wise one, but that is for another time). Why go through the trouble of determining this? Because "It is the government's job" is not, for me at least, a satisfactory answer. To say something should be done simply because we have an institution to do it is fallacious (otherwise, why did the Spanish Inquisition ever end?).

Question:          Why should a government inflict punishment upon criminals beyond simply exiling or ignoring them?

Answer:          In order to ensure the human race's survival the government must punish criminals.

Rationale:          Any criminal act disrupts society to one degree or another, and the more crimes committed, the more society begins to crumble. Unless crime is punished, self interest motivated crime will reduce the human population to groups of less than 150 people. These small groups could easily be killed off by a combination disease, starvation, war, natural disasters, etcetera. Thus the punishment of criminals is in attempt to prevent the human race's extinction.

Assumptions:          The human race is worth keeping around.