Uh.  Government has only one reason of existence, and that IS selfishness.  Since the first kings.  What has evolved, is the way in which government convinces people that they are useful.  The social lie is its cornerstone.  There's no reason to be in government if it is not to be selfish.  It is the violence monopolist that maximizes the profit it can take from that monopoly.  Sometimes, however, you have idiots in power that don't estimate correctly the maximal burden a government can put on its people, and then you get revolutions or invasions.
Ideally, a government squeezes out maximally its population without destroying the illusion of its necessity or its unavoidability, which is what keeps it in place.
The ideal government is like the capable farmer that maximizes the profit he can take from his cattle (the governed people).  And yes, for that, you don't have to be too mean with your cattle before slaughter, on the contrary, you have to "care" about it.