Then as "human concepts" they are purely subjective hang-ups with no metaphysical associations whatsoever. That puts them on the same level as a million other things that mankind has changed its mind about over the years (slavery, discrimination, etc.), which means there's no reason for your "good and evil" to outlast changing cultural mores, popular vote, or whatever powers have the ability to force their wills on the population as a whole. And you're satisfied with this as your basis of "morality?"
The "contract" is unsigned but valid. You are born into it. Those who break it eventually pay, one way or another. Hmm.. not always but often enough that those who reject the "contract" are still held in check. Otherwise we'd have been wiped off the planet long ago.
The "contract" is a myth, the creation of rationalist philsophers of the "18th Century." And all you can do is to invoke police power to prevent all miscarriages of justice (you aren't falling back on "supernatural powers," are you?). But once again, what police power punishes can change with cultural/political mores, etc. If someone advocated decriminalizing murder how would you argue against it? With societal pragmatism? You surely wouldn't start falling back on metaphysical arguments, would you?
BTW, there are have been innumerable miscarriages of justice that have gone undetected over the millenia of human history. No way can your police (or whatever your "one way or another" entails) correct this. But then, since these rules are are merely subjective human hang-ups with no metaphysical reality, it doesn't really matter whether people pay or not, does it?
And why should you care if the human race were wiped off the earth? Would the random, meaningless, self-existent material universe give a darn?