Murder works for whom??? It may work temporarily for the current top dog, but it sustains a system whereby that king is constantly in fear of assassination, and the gov't is left without a good succession policy, and I'm sure that atmosphere must lead to terrible policies in general. Not to mention what kind of example that gives for the society at large.
Silverback gorillas know of no other way of doing things, but humans have the possibility of eventually discovering other ways of doing things. It's here that governmental systems compete on the historical stage, and over the long run humanity approaches something like an objective understanding of the best possible governmental system: some kind of representative democracy, limited in power by a strong constitution. I think drastically different forms of government may survive for long periods of time, but only where the populace is mostly ignorant of the alternatives.
I think history confirms this pattern, but it also simply makes sense that a constitutionally limited representative democracy should be more compatible with human nature than any other system.