There is no moral barrier to human sacrifice and cannibalism until the advent of what truly seems to be advanced Religion. If there is no single God-who-Said then there is no reason why not. Where there is a multiplicity of gods then some of them will require it because paganism is not about living a proper life; it is all about placating capricious gods some of whom are wholly Evil.
By the time Europeans got here sacrifice and cannibalism were already widespread. That these indigenous civilizations got taken down is precisely what needed to happen. May God grant us the wisdom to guide our Western civilization back into His light lest the world becomes the darker and far more brutal place without us.