"We consider that example extreme only because our society has diverged so far from that point. In those times, it would be a perfectly reasonable argument and

"We consider that example extreme only because our society has diverged so far from that point. In those times, it would be a perfectly reasonable argument and the guilt felt would be real." Which only proves that ethics are based on societal norms ("social upbringing"). If you think the norms are bad, try and change them, instead of seeking some mumbo-jumbo "independently justifiable morality", which doesn't even make sense (there is no method of independently justifying a moral position, nor would it convince everyone even if it did it exist. Even human life is not "independently justifiable" sacred. It's only as far as we believe in some (societal) norms). The benefit of this (as opposed to the imaginary "independently justifiable" morals), is that our morals can be changed. This is after all what we call progress, in the societal and moral sense of the word.