Yes, moral propositions are learned. But implicit in your statment about the savagery of those who are not taught morals is the tacit assumption of the transcendent, innate, moral nature of human beings over against that of animals. Binding moral propositions only emanate from authoritative personal sources, and are incumbent only upon persons, not animals.
So if you say that persons do not transcend animals in this regard, which seems to be what you are arguing, then you are cutting off the branch on which you sit. The more you saw, the more you lose of the corresponding transcendent ethic incumbent on all persons to obey. Thus, if you succeed in cutting off the branch, it will no longer make any sense whatever to describe as savages those who are not taught morals, because by cutting off the transcendent nature of man you will also have cut off any transcendent ethic by which to judge and reject the actions of anyone as morally defective, whether it be a sadistic creep playing with a hot cat over burning coals, or a cat toying with a mouse before killing it.
Cordially,