Right, so please explain then, if morality is universal and 'written into human beings' why we have had so much conflict historically? Why do people resist living as subjects of other cultures if morality is the same across culture? And I'd say there are pretty big nuances between societies that claim, for example, that slavery is permissible and those that do not. From those who claim that women have no rights and those that claim women should equal power to men, etc.
Those aren't "nuances".
And even your example of murder, the notion of murder was originally very narrow, coming from a Norse word meaning to kill in secret. Plenty of things we'd consider to be 'murder' today were perfectly permissible in 'ancient civilizations'. Human sacrifice, for one.
"The proof you need is guilt and conscience. "
Guilt and conscience are learned and plenty of people lack both.
"hen how can you condemn what the Nazi's did as wrong"
Right, so we get at the root of the matter. Morality is the metaphysics of the hangman (thanks, Nietzsche). It's just an exercise for people who feel a need to stand in judgment of others. The Nazi actions would not be wrong if they had lived during the time of the ancients. Or hell, even in the time of the Mongols. Wiping out entire cities and everyone in it was more or less acceptable. Read Thucydides (the real one) and his account of the Peloponnesian war, and the slaughter of Melos by the Athenians.
The Nazi actions are considered wrong because they lost the war and modern international opinion is purportedly against brazen attacks on civilians (yet civilians keep being targeted in war).