This supposes a definition of good and just that supercedes God -- that is, some "higher law" that exists outside of all creation and above God.
Yes, that's implied by the idea of a moral God. You do believe god is moral, right? No moral entity puts themselves at the apex of their moral laws. Only an immoral entity is not subject to their own morality.
You can't define God as good and then define good as anything God does. That's circular. If god is good, it's precisely
because he follows a moral code of goodness that's above himself.