Well the answer is clear: there is no absolute moral code. There are however common morals shared by most or all societies, such as prohibitions against murder or rape. These common rules appear to be arrived at independently, in all societies, for reasons of social survival. It would be impossible to maintain a tribal group, for instance, if rape, murder, and theft were allowed internally.

However, these rules seem to apply in certain circumstances, such as internally in a given society, and the same protections will often not be afforded to external groups or individuals. Thus the almost unverversal notion that it is "OK" to kill or steal from foreigners, or outsiders.

Therefore, when a group of poeple is considered sufficiently different to render them not "human", its becomes OK to slaughter them. This can be seen in all times and places, from ancient to modern.

HOWEVER:

There can and should be a higher morality. This would be a morality defined by cetain enlightened individuals, and can transcend the common "mob morality".