Morality is not a product of deductive logic. Morality - as a consistent set of principles to use when evaluating the actions of others & yourself - is a tool we use to maximize human flourishing. As such, the optimal moral systems are discovered using a combination of observation, induction, hypothesis, deduction, and observation again.
For the most part, nobody comes up with successful moral laws through deduction alone. We have to discover them.
Morality isn't really that difficult to develop. Imagine you're Abraham, wandering with your familly, your servants, and your flocks. You're looking for a place to settle down. Would you select a city where they confiscate and distribute all your property? Where they rape your women? Where murder goes unpunished? Hardly. You'd select a city where your people and property are protected. Such cities would grow and prosper. Others would become abandoned ghost towns, inhabited only by bandits. And so it is with nations. The USSR collapsed. The USA has prospered. Others muddle along. To some extent it's a trial-and-error process, but one that a little bit of thinking can pretty much design from scratch.